Sheng-yang Yu1

As I mentioned in my last blog, a cascode MOSFET configuration provides a low-cost alternative for high-voltage applications such as smart meters and motor drives. To further understand how a cascode MOSFET configuration works in high-voltage converters, Figure 1 shows a MOSFET modeled by a switch in parallel with a diode and a capacitor,. In addition to the switch, diode and paralleled capacitor, you must also consider the gate-to-source capacitance, C_g, of the top-side MOSFET.

In this blog post, I will discuss two possible operational conditions when using a cascode MOSFET configuration: V_in < V_Zc and V_in ≥ V_Zc, where V_Zc is the clamping voltage of the Zener diode Z_C.

V_in < V_Zc

When the converter first powers up, capacitor C_C will be charged to V_in through R₁. Once the controller bias voltage is charged above the undervoltage lockout (UVLO) threshold, switch S₁ turns on. As shown in Figure 2, when S₁ turns on, the energy in C_C transfers to C_g and leads to voltage increases on C_g. In order to turn on S₂, the clamping voltage of Z₂ needs to be set higher than the gate-to-source threshold voltage (V_gs(th)) of the top-side MOSFET. It is important to have enough energy in C_g to keep the top-side MOSFET turned on during the entire on state after the bottom-side MOSFET turns on. In other words, C_C can’t be too small. Sometimes, the parasitic capacitance of Z_C might not be enough and will require an external capacitor in parallel with Z_C.

When the MOSFET on the bottom side turns off (Figure 3), the current from the transformer quickly charges C₁. C_g discharges and energy goes back to C_C again. Once C_g discharges to a voltage level lower than V_gs(th), S₂ turns off and C₂ charges up.

V_in ≥ V_Zc

The current-flow directions during the low-side MOSFET turned-on transient when V_in ≥ V_Zc are exactly the same as when V_in < V_Zc. During low-side MOSFET turned-off transient, transformer current discharges C_g and then flows through C_C and Z_C, as shown in Figure 4. In this transient, Z_C acts as a snubber and clamps the low-side MOSFET voltage to V_Zc + V_{F_Z2}, where V_{F_Z2} is the Zener forward voltage drop of Z₂.

The longer turn-off delay time S₂ has after S₁ has already turned off, the higher current flows through S₂ to Z_C, which might lead to a thermal issue for Z_C. Figure 5 below shows a turn-off transient example. As you can see, S₂ is starting to turn off after S₁ is fully turned off. In the transient, S₁ is already off and S₂ is turning on. A surge current flows through Z_C and this causes a thermal issue with Zener diode Z_C.

If the turn-off delay time between S₁ and S₂ can be minimized by using MOSFETs with better transient characteristics (like the example in Figure 6), the current going to Z_C can also be minimized, and reducing  the temperature rise of Z_C.

Many components go into designing high voltage converters with a cascode MOSFET configuration, but mastering the key operations with carefully circuit parameter selection allows for a low cost alternative for high-voltage applications.

Additional Resources

• Discover TI’s Power Tips blog series on Power House.
• Check out the following TI Designs reference design, which use a cascode MOSFET configuration
  • 400V to 690V AC Input, 50W Flyback Isolated Power Supply Reference Design for Motor Drives