Design Example of Isolated Quasi-Resonant Converters Using SiC MOSFET
Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
- AC/DC converter design
- Calculation of current sense resistance
- Capacitor for noise protection
- Capacitor for regulation
- Current sense resistor
- FB pin
- Isolated AC/DC converter design
- Quasi-resonant converter design
- Silicon Carbide
- Zero current detection
This article discusses components necessary for various detection pins used for a power supply IC to perform step-down and regulation control.
Current Sense Resistor：R19
R19 converts the current flowing to the primary side due to the switching transistor Q1 into a voltage. This voltage is monitored by the CS pin, and an output overload protection point is set. The value of R19 is set such that when the primary-side maximum current Ippk (0.66 A) flows, the CS pin overcurrent detection voltage is Vcs = 1 V.
Moreover, the loss P_R19 of the resistor R19 is
Considering pulse resistance, P_R19 is assumed to be 1 W or higher.
Pulse resistance may change with the structure of the resistor and the like even for the same power rating. The manufacturer of the resistor used should be consulted.
Resistor and Capacitor for CS Pin Noise Protection: R22, C13
When noise is not completely eliminated by the blanking function, this RC filter is added. Even when the filter is not necessary, it is recommended that R22 (value approx. 1 kΩ) be inserted as a surge countermeasure. The value of C13 is about 47 pF.
ZT Pin Voltage Setting Resistor: R21
R21 sets the bottom detection voltage at the ZT pin. The bottom detection voltage at the ZT pin is Vzt1 = 100 mV typ. (ZT pin voltage falling) and Vzt2 = 200 mV typ. (ZT pin voltage rising). From ZTOVP(min) = 3.30 V, as a rule Vzt is set to approximately 1 to 3 V. R20 was set to 100 kΩ as per " Switch Setting Resistors for Overload Protection Points". The number of windings is 8 turns for both Nd and Ns, as calculated in "Transformer T1 Design - 2".
The capacitor C11 is used for ZT pin regulation and to adjust the bottom detection timing. It should be set while checking the ZT pin waveform and bottom detection timing. In this circuit example, it is set to 47 pF.
FB Pin Capacitor：C12
C12 is a capacitor for FB pin regulation. A value of approximately 1000 pF to 0.01 μF is recommended. In this circuit example, it is set to 2200 pF.
・Components necessary for the different detection pins should be set according to the data sheet and the design manual.
・If noise enters the detection pins, erroneous operation and other problems may result. The addition of a capacitor or RC filter should be considered.