T
treez
Guest
Hello,
With offline flyback converters, why is there so much support given to the use of slow diodes in the primary RCD clamp network?
I do appreciate that when a slow diode is used, the RCD clamp capacitor no longer has to “swallow” all of the ½ L(lk)I^2 energy contained in the leakage inductor, -because due to the slow turn-off of the slow diode, the clamp capacitor can simply take part in a resonance with the leakage inductance, and indeed some of the energy can be transferred to the output during this resonance, and some can be dissipated in a series resistor. Also, the resonance, which takes place inside the “reverse recovery window” of the slower diode simply dissipates energy in a resistor in series with the clamp diode. As such, you subsequently end up with less voltage across the RCD clamp capacitor, and therefore less power dissipated in the Resistor of the RCD clamp. So I can see the attractions of using a slow diode in the RCD clamps of DCM flyback converters.
However, a slow diode in the RCD clamp is very bad during those moments when the flyback SMPS goes into continuous conduction mode (CCM) …..CCM can occur during output overload or short circuit, or during startup especially if a large output capacitor is used with the flyback SMPS. In CCM, the use of a slow recovery diode in the RCD clamp circuit can be disastrous, with overly large surge currents coming through the diode when the FET switches ON.
So why is everyone clammering to use slow diodes in the Flyback’s RCD clamp circuit? The web is full to the brim of it.
Eg, this app note…
**broken link removed**
- - - Updated - - -
Also, do you agee that if a slow diode is used, then ideally the reverse recovery time of the slow diode should be equal to the period of the resonance between The clamp capacitor and the leakage inductance?
With offline flyback converters, why is there so much support given to the use of slow diodes in the primary RCD clamp network?
I do appreciate that when a slow diode is used, the RCD clamp capacitor no longer has to “swallow” all of the ½ L(lk)I^2 energy contained in the leakage inductor, -because due to the slow turn-off of the slow diode, the clamp capacitor can simply take part in a resonance with the leakage inductance, and indeed some of the energy can be transferred to the output during this resonance, and some can be dissipated in a series resistor. Also, the resonance, which takes place inside the “reverse recovery window” of the slower diode simply dissipates energy in a resistor in series with the clamp diode. As such, you subsequently end up with less voltage across the RCD clamp capacitor, and therefore less power dissipated in the Resistor of the RCD clamp. So I can see the attractions of using a slow diode in the RCD clamps of DCM flyback converters.
However, a slow diode in the RCD clamp is very bad during those moments when the flyback SMPS goes into continuous conduction mode (CCM) …..CCM can occur during output overload or short circuit, or during startup especially if a large output capacitor is used with the flyback SMPS. In CCM, the use of a slow recovery diode in the RCD clamp circuit can be disastrous, with overly large surge currents coming through the diode when the FET switches ON.
So why is everyone clammering to use slow diodes in the Flyback’s RCD clamp circuit? The web is full to the brim of it.
Eg, this app note…
**broken link removed**
- - - Updated - - -
Also, do you agee that if a slow diode is used, then ideally the reverse recovery time of the slow diode should be equal to the period of the resonance between The clamp capacitor and the leakage inductance?