Two diodes change demagnetization-signal polarity
Power-supply designers usually like flyback converters to operate in DCM (discontinuous-conduction mode) rather than in CCM (continuous-conduction mode). In DCM, the flyback converter is a first-order system at low frequencies, which eases the feedback-loop compensation. You can use a low-cost secondary rectifier, thanks to soft blocking conditions. In DCM, IP goes to zero, and the diode stops conducting, whereas the power-switch turn-on event in CCM forces the diode to brutally stop conducting. Also in DCM, valley switching ensures minimum switching losses that COSS and all the parasitic capacitances bring.
In valley switching, or QR (quasiresonant) operation, the curve of the drain-source voltage, VDS, of a typical flyback converter, shows that when the power switch closes, you observe a low level due to the RDS(ON)×IP product (Figure 1a). At the switch opening, VDS rises quickly and starts to ring at a high frequency because of the leakage-inductance presence. During this time, the primary current transfers to the secondary, and a reflected level of N×(VOUT+VF) appears on the MOSFET drain, where N is the secondary-to-primary turns ratio, VOUT is the output voltage, and VF is the secondary-diode forward drop. As soon as the primary current has fallen to zero in DCM operation, the transformer core is fully demagnetized (Figure 1b). The drain branch starts to ring but at a lower frequency than in Figure 1a because the primary inductance, LP, is now involved.
This natural oscillation exhibits the following frequency value, where CLUMP represents all of the circuit's parasitic capacitances, such as COSS and the stray capacitance from the transformer.
As with any sinusoidal signal, there are peaks and valleys. When you restart the switch in the valley, all the parasitic capacitance values are at their lowest possible levels. Also, the capacitive losses, which are equal to 1/2×CLUMP×VDS2×FSW, are small because the MOSFET is no longer the seat of turn-on losses, which removes the usual turn-on parasitics. That is the secret of QR operation.
You can easily observe the core flux through an auxiliary winding (Figure 2a). Thanks to the coupling between the windings, the auxiliary section delivers a voltage image of the core's flux through the following formula:
Now, you can wire the winding either in flyback operation, as the power winding, or in forward operation. The observed signals look the same but have different polarity (Figure 2b). Note that both signals center about ground. The problem lies in the fact that most PWM controllers accept only the flyback polarity. Typical examples include the MC33364 and MC44608 (http://www.onsemi.com). In battery-charger applications, you usually wire the auxiliary winding—the one that self-supplies the controller and gives the demagnetization signal—in forward mode. The reason is simple: When the battery you charge is close to 0V, the auxiliary windings are also nearly 0V because both windings are coupled in flyback mode. By operating in forward mode, whatever happens on the secondary side is invisible, and the voltage is always there to supply the controller. However, the demagnetization signal now has the wrong polarity, and the controller doesn't restart at the core's reset event.
Figure 3a shows a way around this problem. You still wire the winding for forward operation, but you add two extra diodes in series with the winding. At the switch closing, you apply N×VHV, where N is the ratio between the auxiliary winding, NA, and the primary winding, NP. You clamp VDEM to –0.6V, and the current circulates through RVALLEY. At the switch opening, the voltage reverses and becomes positive but clamped to 0.6V on VDEM. When this level collapses, the PWM controller reactivates the power switch.
You can implement this same type of circuit for PWM controllers that need a forward demagnetization signal but for which you would like to operate the auxiliary winding in flyback mode (Figure 3b). The problem and the cure are similar.
When you properly select RVALLEY, this resistance naturally combines with sense-pin internal capacitance to add switch delay right in the middle of the wave (Figure 4).
Some controllers exhibit different demagnetization threshold levels. The MC33364 starts at around 1V, and the MC44608 toggles at 65 mV. Because of the diodes, you clamp VDEM between ±600 mV, which could not trigger the MC33364. A small offset from the internal reference to the demagnetization pin brought by a 150-kW resistor and a typical RVALLEY of 10 kW have provided good circuit operation.