Clamp voltage detection and over-voltage protection for power supply topologies

The present disclosure relates to power converters and more particularly to power converters using active clamp flyback topologies.

Flyback converters are often used in alternating current to direct current (AC/DC) conversion. In some flyback architectures, an active clamp capacitor in series with a synchronously driven high side transistor is connected in parallel to the primary winding of the flyback transformer to smooth the output of the converter. The resulting converter is known as an active clamp flyback (ACF) converter.

When the ACF converter operates in steady state, it maintains the clamp voltage across the clamp capacitor (V CLAMP ) at approximately a certain value but with ripple around this value due to charging and discharging caused by leakage inductance current. If a fault happens that prevents the high side transistor from switching, the ACF converter continues to operate like a conventional flyback converter. However the MOSFET has a parasitic body diode that results from connecting the body to the source terminal. This body diode causes V CLAMP to increase since there is no discharge path. This buildup in voltage across the clamp capacitor will eventually cause the ACF converter to be damaged or otherwise to fail.

The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings, in which:

FIG. 1 illustrates in partial block diagram and partial schematic form a power supply system according to various embodiments;

FIG. 2 illustrates a set of timing diagrams showing several signals of the power supply system of FIG. 1 ;

FIG. 3 illustrates in partial block diagram and partial schematic form a simplified circuit of an ACF controller that may be used to implement the V CLAMP detection circuit and the OVP circuit of FIG. 1 ;

FIG. 4 illustrates a flow chart of the operation of the ACF controller of FIG. 1 ; and

FIG. 5 illustrates in partial block diagram and partial schematic form an ACF controller that can be used to implement the ACF controllers of FIGS. 1 and 3 .

The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.

In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the circuits described herein can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.

FIG. 1 illustrates in partial block diagram and partial schematic form a power supply system 100 according to various embodiments. Power supply system 100 is an offline, AC/DC converter having an ACF topology. Power supply system 100 includes generally an AC voltage source 110 , a bridge rectifier 120 , a bulk capacitor 130 , a flyback transformer 140 , a primary side circuit 150 ((It's not indicated in FIG. 1 .)), a secondary side circuit 160 , a bias winding circuit 170 , and an ACF controller 180 .

AC voltage source 110 has first and second terminals and provides a time-varying full-wave AC voltage. For example, AC voltage source 110 could be a 110 V RMS , 60 hertz (Hz) AC mains or a 220 V RMS , 50 Hz AC mains.

Bridge rectifier 120 includes diodes 122 , 124 , 126 , and 128 . Diode 122 has an anode connected to the first terminal of AC voltage source 110 , and a cathode. Diode 124 has an anode connected to primary ground, and a cathode connected to the anode of diode 122 . Diode 126 has an anode connected to the second terminal of AC voltage source 110 , and a cathode connected to the cathode of diode 122 . Diode 128 has an anode connected to primary ground, and a cathode connected to the anode of diode 126 .

Bulk capacitor 130 has a first terminal connected to the cathodes of diodes 122 and 126 , and a second terminal connected to primary ground.

Flyback transformer 140 is a magnetic core transformer including a primary winding 141 , a secondary winding 142 , and an auxiliary winding 143 . Also shown in FIG. 1 are a leakage inductance 144 and a magnetizing inductance 145 . Primary winding 141 has a first end coupled to the cathodes of diodes 122 and 126 through leakage inductance 144 , a second end coupled to the first end through magnetizing inductance 145 , and has a number of turns labeled “N P ”. Secondary winding 142 has a first end, and a second end connected to secondary ground. Auxiliary winding 143 has a first end, and a second end connected to primary ground.

Primary side circuit 150 includes a low side switch 151 , a high side switch 152 , and a clamp capacitor 153 . Low side switch 151 is implemented as an N-channel metal-oxide-semiconductor field effect transistor (MOSFET) having a drain connected to the second end of primary winding 141 , a gate, and a source connected to primary ground. High side switch 152 is implemented as an N-channel MOSFET having a drain connected to a node labeled “A”, a gate, and a source connected to the drain of low side switch 151 and to the second end of primary winding 141 . Clamp capacitor 153 has a first terminal connected to the cathodes of diodes 122 and 126 , and a second terminal connected to the drain of high side switch 152 . In some embodiments, a Zener diode (not shown) may be connected in parallel to clamp capacitor 153 , whose anode is connected to the cathodes of diodes 122 and 126 and whose cathode is connected to Node “A.” Associated with low side switch 151 and high side switch 152 are respective parasitic diodes 154 and 155 . These parasitic diodes are known as body diodes and represent the PN junctions formed by the connection of the respective MOSFET transistor's body to its source. Each of the parasitic diodes has an anode connected to the source of the respective MOSFET, and a cathode connected to the drain of the respective MOSFET.

Secondary side circuit 160 includes a diode 162 and a capacitor 164 . Diode 162 has an anode connected to the first end of secondary winding 142 , and a cathode for providing an output voltage labeled “V OUT ” to a load (not shown in FIG. 1 ). Capacitor 164 has a first terminal connected to the cathode of diode 162 , and a second terminal connected to secondary ground.

Bias winding circuit 170 includes a diode 172 , a capacitor 174 , a resistor 176 , and a resistor 178 . Diode 172 has an anode connected to the first end of auxiliary winding 143 , and a cathode. Capacitor 174 has a first terminal connected to the cathode of diode 172 , and a second terminal connected to primary ground. Resistor 176 has a first terminal connected to the first end of auxiliary winding 143 , and a second terminal. Resistor 178 has a first terminal connected to the second terminal of resistor 176 , and a second terminal connected to primary ground.

ACF controller 180 has a high voltage terminal 181 labeled “HV” connected to the drain of high side switch 152 and to the second terminal of clamp capacitor 153 , a high side switch gate terminal 182 labeled “HG” connected to the gate of high side switch 152 , a low side switch gate terminal 183 labeled “LG” connected to the gate of low side switch 151 , a power supply terminal 184 labeled “V DD ” connected to the cathode of diode 172 and to the first terminal of capacitor 174 , a sense voltage terminal 185 labeled “V S ” connected to the second terminal of resistor 176 and to the first terminal of resistor 178 , and a ground terminal 186 labeled “GND” connected to primary ground.

ACF controller 180 includes two circuits of interest, a V CLAMP detection circuit 187 , and an over-voltage protection (OVP) circuit 188 . ACF controller 180 also includes various other control and protection terminals and circuits. Since these are conventional, they are not shown nor described in detail herein.

In operation, power supply system 100 receives a time varying input voltage from AC voltage source 110 and generates output voltage V OUT to supply a current based on the power demand of a load receiving output voltage V OUT . While power supply system 100 uses an AC/DC ACF topology, it should be understood that the techniques disclosed herein may be implemented in DC/DC active clamp topologies, AC/DC active clamp forward topologies, DC/DC active clamp forward topologies, etc. The circuitry described herein with reference to power supply system 100 may reside, in whole or in part, within a single device such as, for example, an integrated circuit (IC), or alternatively, some or all of the elements in power supply system 100 may be discrete components, combinations of ICs and discrete components, etc. Power supply system 100 may be utilized to convert conventional wall power (e.g., 110 V RMS AC, 220 V RMS AC, etc.) to a DC voltage for a mobile communication and/or computing device.

ACF controller 180 controls the duty cycle of switch 151 using pulse width modulation (PWM) to regulate V OUT and adjust the duty cycle of switch 151 using conventional voltage and/or current mode control. In addition, ACF controller 180 activates high side switch 152 in a generally complementary fashion to activating low side switch 151 . ACF controller 180 thereby operates power supply system 100 in a repeating sequence of switching states, as is well understood.

ACF controller 180 uses the active clamp formed by high side switch 152 and clamp capacitor 153 to clamp the transistor turn-off voltage spike and to recycle the transformer leakage energy. This operation makes zero-voltage-switching (ZVS) possible for both low side switch 151 and high side switch 152 , while requiring a few additional power stage components and slightly increasing the complexity of ACF controller 180 compared to traditional flyback control.

Unlike conventional ACF controllers, however, ACF controller 180 connects the HV pin to node A both to measure V CLAMP and to allow the controllable discharge of V CLAMP . Thus, ACF controller 180 prevents the uncontrolled buildup of V CLAMP when high side switching fails. As will be described further below, ACF controller 180 also does not measure V CLAMP directly, avoiding the need for an additional integrated circuit high-voltage terminal. Instead, it measures V CLAMP indirectly by measuring the voltage on node A and using the current through V S terminal 185 to calculate the voltage on the first terminal of clamp capacitor 153 . ACF controller 180 also discharges V CLAMP to a safe level using a junction field effect transistor (JFET).

Moreover, ACF controller 180 uses an existing IC terminal—the HV terminal—to measure the voltage on the second terminal of clamp capacitor 153 and to perform the discharge operation. ACF controller 180 also connects the HV pin to an internal bypass circuit during startup to provide power to ACF controller 180 before auxiliary winding 143 and bias winding circuit 170 are able to generate a sufficiently large value for V DD .

ACF controller 180 determines V CLAMP as follows. First, ACF controller 180 determines an initial peak value of input voltage V IN (t), referred to herein as V IN *, at startup before switching using switches 151 and 152 . At this point, clamp capacitor 153 is assumed to be fully discharged, and therefore V IN * represents the isolated, initial peak value of V IN (t).

Second, after startup, ACF controller 180 starts switching switches 151 and 152 , measures the value of the current into the V S pin, labeled “I VS (t)”, and also measures the peak value I VS *. The current into V S terminal 185 is proportional to V IN (t), and the value of I VS * is also proportional to V IN *, as shown by the following equation, in which N A is the number of turns of auxiliary winding 143 , N P is the number of turns of primary winding 141 , and R 1 is the resistance of resistor 176 :

I VS * = V TH - N A N P ⁢ V I ⁢ ⁢ N * R 1 [ 1 ]

Assuming V TH is approximately zero volts, I VS * is given by:

I VS * = - N A N P ⁢ V I ⁢ ⁢ N * R 1 [ 2 ]

Thus the peak current I VS * through V S pin 185 is proportional to the peak input voltage V IN *. ACF controller 180 may include memory, not shown in FIG. 1 , to store the values of V IN * and I VS *.

Third, during normal operation, ACF controller 180 measures and stores the instantaneous value of I VS (t).

Fourth, ACF controller 180 calculates V IN (t) using the measured values of I VS (t), V IN *, and I VS *. V IN (t) is given by the following equation:

V IN ( t )= I VS ( t )· V/I VS *  [3]

Furthermore ACF controller 180 by measuring the voltage of node A, which is equal to V IN (t)+V CLAMP , and subtracting V IN (t) from the quantity V IN (t)+V CLAMP :

V CLAMP =V A −V IN ( t )= V A −I VS ( t )· V IN */I VS *  [4]

In this manner, ACF controller 180 uses OVP circuit 188 to detect when the value of V CLAMP is too high, and to selectively discharge V CLAMP to a value within the desired range. OVP circuit 188 does this function hysteretically. In one example, in a 120 V RMS AC mains system, OVP circuit 188 detects when the calculated value of V CLAMP rises above 180 V, and then discharges V CLAMP until the voltage falls below 120 V.

FIG. 2 illustrates a set of timing diagrams showing several signals of power supply system 100 of FIG. 1 . In timing diagram set 200 , the horizontal axis represents time in milliseconds (msec), and the vertical axis represents the value of several signals in volts or amperes, as the case may be. Timing diagram set 200 shows five signals of interest: V IN (t), HV ON V DD , LG, and I VS (t) in corresponding timing diagrams 210 , 220 , 230 , 240 , and 250 . Timing diagram set 200 also shows four time of interest, labeled “t 0 ”, “t 1 ”, “t 2 ”, and “t 3 ”.

Timing diagram 210 includes a waveform 212 of V IN (t) shown as a rectified sine wave, i.e. a haversine wave, formed from the operation of bridge rectifier 120 of FIG. 1 . This haversine wave can be generated, for example, as a full wave rectified AC mains voltage. A waveform 214 represents a smoothed haversine wave through the operation of bulk capacitor 130 . Waveform 214 reaches a peak value labeled “V IN *” between t 0 and t 1 . time t 1 , ACF controller 180 starts switching current through primary winding 141 of flyback transformer 140 to deliver current to the load. Waveform 214 starts to follow more the contours of waveform 212 after time t 1 due to the buildup of magnetic flux in flyback transformer 140 .

Timing diagram 220 includes a waveform of a control signal labeled “HV ON ” having an active high puke 222 for more than one full cycle of V IN (t) between t 0 and t 1 until V DD reaches its threshold to activate the ACF controller 180 . After t 1 , however, ACF controller 180 only activates the HV ON signal for very short time periods. For example, ACF transformer may activate HV ON for 3 microseconds (μs) for every 300 μs.

Timing diagram 230 includes a waveform 232 of operating power supply voltage V DD . During startup and before ACF controller 180 starts switching flyback transformer 140 , a bypass circuit internal to ACF controller 180 uses the primary line voltage to power the internal circuits. Between times t 0 and t 1 , ACF controller 180 uses the smoothed, rectified input voltage to ramp the voltage on the external storage capacitor in a roughly linear mode. Once V DD reaches an internal lockout threshold, labeled “V UVLO ”, however, the bypass circuit disconnects the smoothed, rectified input voltage from the storage capacitor. Thereafter, the voltage initially drops as the current demand is greater than the ability of the auxiliary winding to provide current. Over a relatively short amount of time, the voltage on the capacitor ramps back toward the voltage generated by auxiliary winding 143 .

Timing diagram 240 includes a waveform 242 showing the value of the LG signal applied to the gate of switch 151 . As is shown by waveform 242 , it starts in an inactive state between times t 0 and t 1 , keeping switch 151 non-conductive. After time t 1 , waveform 242 switches at a relatively high rate compared to the frequency of waveform 212 , and thus it is shown as a shaded area.

Timing diagram 250 includes a waveform 252 showing the value of the I VS (t) signal conducted through V S terminal 185 . Waveform 252 starts switching at time t 1 , and reaches a peak value labeled “I VS *” during the period between time t 1 and time t 2 . Because it is the next subsequent cycle, this technique assumes that the line voltage has not changed significantly.

The inventors discovered that the instantaneous value of the line voltage, i.e. the voltage across bulk capacitor 130 , can be determined without measuring it directly. The voltage across the auxiliary winding 143 , and thus signal I VS , is proportional to the line voltage while low side switch 151 turns on. If the value of the peak voltage V IN * is multiplied by the ratio of the value of the instantaneous current I VS (t) to the peak current I VS *, then the value of the instantaneous voltage V IN (t) at the first terminal of clamp capacitor 153 can be calculated without directly being measured. The instantaneous value of V CLAMP can be calculated as the difference between the instantaneous voltages on the first and second terminals of clamp capacitor 153 . Thus ACF controller 180 does not need an additional IC high-voltage terminal to measure V IN (t) directly, reducing the cost of ACF controller 180 .

Once V CLAMP has been determined, then ACF controller 180 can use it to selectively implement an overvoltage protection function to prevent voltages that are so high that they might otherwise cause damage to the components of power supply system 100 . For example, ACF controller 180 can perform the overvoltage protection function by selectively discharging the voltage on the second terminal of clamp capacitor 153 . An overvoltage protection technique that takes into account green mode and other low power modes is described further below.

FIG. 3 illustrates in partial block diagram and partial schematic form a simplified diagram of an ACF controller 300 that may be used to implement V CLAMP detection circuit 187 and OVP circuit 188 of FIG. 1 . ACF controller 300 includes HV terminal 181 , a switch 310 , a resistor 320 , a transconductance amplifier 330 , and a comparator 340 . Switch 310 has a first terminal connected to H V terminal 181 , a second terminal, and a control terminal. Resistor 320 has a first terminal connected to the second terminal of switch 310 , and a second terminal connected to ground. Transconductance amplifier 330 has a non-inverting input for receiving a voltage labeled “V IN CAL ”, an inverting input connected to primary ground, a positive current terminal connected to V DD , and an output terminal connected to the first terminal of resistor 320 . Comparator 340 has a non-inverting input connected to the first terminal of resistor 320 , an inverting input for receiving a threshold voltage labeled “V TH ”, and an output for providing an output signal labeled “OVP” shown in FIG. 3 as being connected to the control terminal of switch 310 .

In operation, ACF controller 300 periodically closes switch 310 to sample the voltage on the HV pin. In one example, ACF controller 300 closes switch 310 once for 3 μs during a period of 300 μs. The voltage on the second terminal of clamp capacitor 153 that is connected to the HV pin is equal to V IN (t)+V CLAMP . V IN (t) can be measured directly, but would require the addition of an integrated circuit high-voltage terminal. However ACF controller 300 uses the indirectly measured signal V IVS to measure V CLAMP and to perform a protection operation when V CLAMP is too high.

The current flowing through switch 310 into the first terminal of resistor 320 is equal to (V IN (t)+V CLAMP )/R S , in which R S is the resistance of resistor 320 . Transconductance amplifier 330 subtracts a current equal to V IN CAL *g m from this current, in which g m is the transconductance of transconductance amplifier 330 . Note that since V CLAMP +V IN (t)>V DD , the current flows from HV terminal 181 into V DD and reduces the current through resistor 320 . The current flowing through resistor 320 is equal to (V IN (t)+V CLAMP )/R S −V IN CAL *g m . If g m , is set to be equal to 1/R S , then the current through resistor 320 is equal to (V IN (t)+V CLAMP −V IN CAL )/R S . If V IN (t)≈V IN CAL , then the voltage on the non-inverting input of comparator 340 is approximately equal to V CLAMP . By proper selection of g m , R S , and V TH , comparator 340 can activate the OVP signal when V CLAMP is above a certain undesirable voltage level.

In response to the activation of the OVP signal, switch 310 closes and starts to discharge clamp capacitor 153 . Once the voltage across clamping bulk capacitor 130 falls to a sufficiently low voltage, ACF controller 300 deactivates the OVP signal. For example, ACF controller 300 can activate the OVP signal if V CLAMP is greater than 180 volts, and then subsequently de-active the OVP signal if V CLAMP falls below 120 volts.

This simple example shows how ACF controller 180 calculates V IN (t) without using an extra integrated circuit high-voltage terminal, and protects clamp capacitor 153 when V CLAMP is too high. In some embodiments, ACF controller 180 includes other advantageous mechanisms to support green mode and to reduce power consumption in a manner that will now be explained in greater detail.

FIG. 4 illustrates a flow chart 400 of the operation of ACF controller 180 of FIG. 1 . In a decision box 402 , ACF controller 180 determines whether it is in a green mode by determining whether a green mode signal active, i.e. in this example whether the green mode signal is in a logic high state. If the green mode is active, ACF controller 180 turns off switch 310 in an action box 404 and flow returns to decision box 402 , thus keeping switch 310 off while ACF controller 180 remains in green mode and reducing power consumption. Since there is no switching in green mode, there is no mechanism for V CLAMP to continue to increase, and thus ACF controller 180 saves power in green mode.

If ACF controller 180 is not in green mode, for example in a normal mode or in a burst mode, then at action box 406 ACF controller 180 activates switch 310 for a first time period, e.g. 3 μs, to monitor V CLAMP . ACF controller 180 can perform this monitoring at regular intervals (e.g., 3 μs during a period of 300 μs) or at random intervals. In decision box 408 , ACF controller 180 determines whether V CLAMP is greater than a first threshold, e.g. 180 V. If V CLAMP is greater than the first threshold, then at action box 410 ACF controller 180 keeps switch 310 closed while ACF controller 180 continues to monitor V CLAMP . The timer counts for a second time period, e.g. 500 μs. During this time, clamp capacitor 153 discharges to V DD capacitor 174 and at the end of this time period, ACF controller 180 is able to measure V CLAMP as described above to determine whether the buildup in V CLAMP was a temporary occurrence or a more serious condition.

Flow proceeds to a set of three decision boxes 412 , 414 , and 420 . In decision box 412 , ACF controller 180 determines whether V CLAMP has discharged below a lower threshold, e.g. 120 V. If V CLAMP has fallen below the first threshold, then ACF controller 180 concludes that high side switch 152 is working properly and flow returns to decision box 402 . If not, flow proceeds to decision box 414 .

In decision box 414 , ACF controller 180 determines whether the timer has expired. If so, then ACF controller 180 determines whether V CLAMP is still above the first threshold, e.g. 180 V. If so, then ACF controller 180 concludes that switch 152 or the connection between ACF controller 180 and switch 152 is faulty and performs on OVP operation in action box 418 . In one example of the OVP operation, ACF controller 180 deactivates both switches 151 and 152 and otherwise shuts down power supply system 100 to protect the system and the load. If not, then ACF controller 180 concludes that switch 152 is probably working properly and returns the flow to decision box 402 . For example, the buildup in V CLAMP may be the result of ACF controller 180 being in a burst mode, and the 500 μs interval would be long enough to lower V CLAMP below 180 V.

If the timer has not expired, then flow proceeds to decision box 420 . In decision box 420 , ACF controller 180 determines whether the green mode has become active. Since the activation of the green mode was previously checked in decision box 402 , this condition indicates that the green mode was entered only after V CLAMP was detected above 180 V. If the green mode signal is not active, then ACF controller returns flow to decision box 412 and ACF controller 180 remains in this loop until either V CLAMP falls below 120 V, the timer has expired, or green mode is entered.

If green mode has been entered after the 500 μs timer was started but before it expired, then flow continues to action box 422 . In action box 422 , ACF controller 180 keeps switch 310 closed and resets, but does not restart, the 500 μs timer. Green mode automatically stops the switching of switches 151 and 152 , and thus there is no longer a mechanism to increase the voltage on V CLAMP further. In decision box 424 , ACF controller 180 determines whether V CLAMP has fallen below 120 V. If V CLAMP has fallen below 120 V, then flow returns to decision box 402 , and ACF controller 180 continues to execute decision box 402 and action box 404 until the end of green mode as described above. If however V CLAMP is not below 120 V, then flow continues to decision box 426 . In decision box 426 , ACF controller 180 determines whether green mode is still active. If the green model signal is still high, i.e. green mode is still active, then flow returns to action box 422 . If the green mode signal has gone low, indicating an end to green mode, then flow returns to action box 410 and the flow proceeds as described above.

In various embodiments, ACF controller 180 can implement the flow shown in flow chart 400 using hardware circuitry, firmware running on a microcontroller, or some combination of the two. ACF controller 180 implemented the operation in flow chart 400 to protect power supply system 100 from very high voltages related to the switching of switch 152 that can cause a runaway increase in V CLAMP .

FIG. 5 illustrates in partial block diagram and partial schematic form an ACF controller 500 that can be used to implement ACF controller 180 of FIG. 1 and ACF controller 300 of FIG. 3 . ACF controller 500 can be implemented in a single integrated circuit but can also be implemented with discrete components. ACF controller 500 includes specific circuit implementations of V CLAMP detection circuit 187 and OVP circuit 188 connected to HV terminal 181 , V DD terminal 184 , and V S terminal 185 , as well as a bypass circuit 590 .

V CLAMP detection circuit 187 includes a junction field effect transistor (JFET) 510 , a resistor 520 , a transconductance amplifier 530 , peak detectors 540 and 550 , and a calculation circuit 560 . J FET 510 has a drain connected to HV terminal 181 , a gate, and a source. In other embodiments, JFET 510 could be replaced by another type of transistor such as a depletion MOSFET. Resistor 520 has a first terminal connected to the source of JFET 510 , and a second terminal connected to ground. Transconductance amplifier 530 has a non-inverting input for receiving the V IN CAL signal, an inverting input connected to primary ground, and an output connected to the first terminal of resistor 520 . Peak detector 540 has an input connected to the first terminal of resistor 520 , a control input for receiving a signal labeled “HV STARTUP”, and an output for providing signal V IN *. Peak detector 550 has an input connected to V S terminal 185 , a control input for receiving a signal labeled “SAMPLE”, and an output for providing the I VS * signal. Calculation block 560 has a first input connected to the output of peak detector 540 , a second input connected to the output of peak detector 550 , a third input connected to terminal 185 and an output connected to the non-inverting input of transconductance amplifier 530 for providing signal V IN CAL thereto.

OVP circuit 188 includes a comparator 570 and a timing and control block 580 . Comparator 570 has a non-inverting input connected to the first terminal of resistor 520 for receiving the V CLAMP signal, an inverting input for receiving the V TH reference voltage, and an output. Timing and control block 580 has an input connected to the output of comparator 570 , a first output connected to the gate of JFET 510 , and a second output connected to the input of peak detector 550 for providing the SAMPLE signal thereto.

Bypass circuit 590 includes a switch 592 and a comparator 594 . Switch 592 has a first terminal connected to the first terminal of resistor 520 , a second terminal connected to V DD terminal 184 , and a control terminal. Comparator 594 has a non-inverting input for receiving a reference voltage labeled “V UVLO ”, an inverting input connected to the second terminal of switch 592 , and an output connected to the control input of peak detector 540 and to the control terminal of switch 592 for providing the RV STARTUP signal thereto.

ACF controller 500 is an embodiment of an ACF controller that implements the operations of the ACF controller described in FIGS. 1-4 above. Upon startup, timing and control block 580 activates the signal on the gate of JFET 510 . Since V DD is low (assuming capacitor 174 is fully discharged), comparator 594 activates the HV STARTUP signal in a high state, closing switch 592 and allowing capacitor 174 to be charged through the HV pin. Peak detector 540 also captures the value of the HV pin at startup, V IN *, since the voltage across clamp capacitor 153 is initially zero. As soon as the voltage on V DD terminal 184 exceeds V UVLO , comparator 594 deactivates the HV STARTUP signal, opening switch 592 . Thus ACF controller 500 has HV terminal 181 to provide internal power during startup, and also uses it further to measure V CLAMP +V IN (t) after startup.

Timing and control block 580 activates the SAMPLE signal during the first cycle after the startup period as shown in FIG. 2 , and peak detector 550 captures the peak value of the primary current, I VS *, after timing and control block 580 deactivates the SAMPLE signal. Calculation block 560 further calculates the value of V IN CAL by first determining V IN CAL as described above, and then scaling V IN CAL by g m and R S . Timing and control block 580 opens and closes the switch formed by JFET 510 according to the flow chart of FIG. 4 in hardware, and detects whether V CLAMP is above the high threshold or below the low threshold as described above. As shown in FIG. 5 , comparator 570 is a hysteretic comparator, but in other embodiments it could be implemented with separate comparators having corresponding fixed thresholds.

Thus while specific embodiments have been provided, many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure. For example, while ACF controller 180 can be implemented as a single monolithic integrated circuit, in other embodiments different portions of ACF controller 180 can be implemented either monolithically or as discrete components. The high-side and low-side switches were shown as being N-channel MOSFETs, but in other embodiments other transistor types may be used. Likewise, switch 310 can be implemented with a JFET as shown in FIG. 5 , or with other transistor types such as depletion mode MOSFETs. Moreover the protection mechanisms described herein may be changed or altered for different applications.

The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments that fall within the scope of the claims.