Can I use an SR flip to control an active clamp flyback control?
I am not very sure which method is a good choice to control the Qaux and Q1.
Could you please provide some suggestions?
\$\begingroup\$
\$\endgroup\$
Add a comment
|
2 Answers
\$\begingroup\$
\$\endgroup\$
4
The active-clamp flyback (ACF) has gained popularity with the high-density USB 3.0 chargers. Owing to its zero-voltage switching (ZVS) on the main power transistor, the designer can push the switching frequency at 200-300 kHz and shrink the magnetics. The control of the upper MOSFET requires the insertion of a deadtime for properly controlling the switching events and ensure ZVS:
When the controller instructs the main power switch to turn off, the voltage immediately rises and the upper-switch body diode conducts. Then, after a few hundreds of ns, the upper-side switch is turned on in ZVS.
When the controller initiates a new switching cycle, it turns off the upper-side switch which was conducting in quadrant III. Both switches are now off for the deadtime duration. The current now depletes the parasitic capacitance lumped at the drain and near- or full-ZVS is obtained on the main switch which can now be turned on.
So you see the need for an adequate deadtime for the driving signals. See below my model excerpted from the 60+ free SIMPLIS templates that go along with my last book on switching transfer functions:
When you run the simulation, you can see the presence of the deadtime between the transition as well as the decrease of the drain voltage before turn-on:
In some cases like with the forward converter, the deadtime can be adjusted on the fly to cope with the various loading conditions and ensure near-ZVS across a wide range of operating conditions.
answered Aug 17, 2022 at 17:08
-
\$\begingroup\$ In the bottom right section of the schematic there's a mysterious
R4with an initial condition and a hint of nonlinearity (and marked polarity). What element is that? Also, sorry for the nitpicking, wouldn't it have been easier to give up the inverter (U2) and use theQNoutput of the SR latch? Or is theQNoutput only cosmetic? \$\endgroup\$ Commented Aug 17, 2022 at 17:45 -
\$\begingroup\$ Hi @Verbal Kint, I also have the same questions as a concerned citizen said. and what's the U4 function in this circuit? Can I calculate the ZVS timing? \$\endgroup\$– Power JJCommented Aug 18, 2022 at 0:43
-
1\$\begingroup\$ @aconcernedcitizen, component \$R_4\$ in the schematic represents a PWL voltage-dependent resistance: you describe the current flowing in the component based on the applied voltage across its terminals. You can therefore model segments of a diode in a forward or reverse mode. Here, it is a simple Zener diode. Look at slide 14 in this seminar. Regarding \$Q_b\$, oui, I could have saved an inverter, good point : ) \$\endgroup\$ Commented Aug 18, 2022 at 6:45
-
\$\begingroup\$ @VerbalKint Aha, got it! It helps if you're familiar with a program... ;-) \$\endgroup\$ Commented Aug 18, 2022 at 9:28
\$\begingroup\$
\$\endgroup\$
1
About Active clamp flyback control, can I use SR flip to control it?
Sure you can but, will it make any improvement, no.
Look at the body diode symbol in the MOSFET; it will naturally conduct when excess energy from the flyback leakage inductance is present so, the MOSFET is not needed. OK you may save about 0.5 watts in power dissipation compared to a diode but, the extra-complexity to drive the MOSFET will waste at least that power or more.
Far, far too complex to drive that MOSFET and, no clear returns that justify it.
You also need a resistor in parallel with the clamping capacitor.
answered Aug 17, 2022 at 15:56
-
\$\begingroup\$ Hi @Andy aka, Only using SR can't achieve the ZVS, do you mean that? \$\endgroup\$– Power JJCommented Aug 18, 2022 at 0:44