There are three components in the application circuit to help the depletion MOSFET (Q_S) perform ZVS sensing safely: C_SWS, R_SWS, and D_SWS. Design considerations and selection guidelines for the values of these components are given here.

At the rising edge of the switch-node voltage, the fast dV/dt may couple through the drain-to-source capacitance of Q_S (C_OSS(Qs)) and generate a charge current that flows into the circuit loading on the Q_S source pin. The result may be a possible voltage overshoot on both the SWS pin and across the gate-to-source of Q_S (V_GS(Qs)) because the gate is tied to P13. The SWS pin, having an absolute maximum voltage rating of 38 V, can handle higher voltage stress than V_GS(Qs). Therefore, carefully select a capacitor (C_SWS) between the SWS pin and GND to prevent the voltage overshoot from damaging the Q_S gate. Because C_OSS(Qs) and C_SWS form a voltage divider, the minimum C_SWS (C_SWS(min)) can be derived as

where

• V_{GS_MAX(Qs)} is the de-rated maximum gate-to-source voltage of Q_S
• V_P13 is the steady-state voltage level of 13 V

Without resistive damping, both the charge current on the rising edge of V_SW and the discharge current on the falling edge of V_SW may oscillate with the parasitic series inductance within the ZVS sensing network resonating with C_SWS. Therefore, a series resistor (R_SWS) between SWS pin and source-pin of Q_S is used to dampen any high-frequency ringing, helping to obtain a cleaner sensing signal on the SWS pin and preventing any high-frequency current from interfering with other noise-sensitive signals. R_SWS can be expressed as:

Equation 37.

where

• L_SWS is the lumped parasitic inductance including the packaging of Q_S and PCB traces of Q_S and C_SWS return path
• C_Dz is the junction capacitance of any zener diode applied across C_SWS, if used (usually not necessary).

For most applications, use a resistor value slightly higher than 500 Ω. The resistor and a 22-pF ceramic capacitor between the SWS pin and the bulk input capacitor ground form a small sensing delay to help the internal detection circuit to identify the ZVS characteristic correctly.

Based on the above design guide, even though R_SWS and C_SWS may be sufficient to manage the voltage overshoot in normal operation, a low-capacitance bi-directional TVS diode (D_SWS) across BSS126 gate and source is highly recommended to serve as a safety backup of the ZVS sensing network. Regular Zener diodes are not suitable due to high capacitance and slow clamping response. Ensure the clamping voltage of D_SWS is less than BSS126 voltage rating but greater than 15 V.

A general recommendation is to use a 50-V 22-pF C0G-type ceramic capacitor for C_SWS, a 510-Ω chip resistor for R_SWS, and a bi-directional TVS diode with clamp voltage of 18 V for D_SWS. Too large of R_SWS or C_SWS introduces a sensing delay between the actual V_SW and the SWS pin, causing the ZVS control to unnecessarily extend t_DM in order to pull down V_SW earlier than expected before the end of t_Z . As shown in Figure 7-5, the larger R_SWS is, the smaller supply current to charge the VDD capacitor. If the reduced charge current (I_SWS) is lower than the total consumed current from the controller (I_START) and from the external circuitry on the VDD and P13 pins, V_VDD may not be able to reach V_VDD(ON) and the controller can not initiate any switching event.