Applications of Depletion-Mode MOSFETs in Various Circuits and Systems

Power MOSFETs are most commonly used in switching applications, where they serve as switches. However, in certain applications such as start-up circuits in SMPS, surge and overvoltage protection, reverse connection protection, or solid-state relays, power MOSFETs need to operate as normally “open” switches when the gate-to-source voltage (VGS) is zero. Power MOSFETs that operate as normally “open” switches at VGS = 0V are called depletion-mode MOSFETs.

The main difference between enhancement-mode (EM) and depletion-mode (DM) MOSFETs is as follows:

The first significant difference is illustrated in the circuit diagrams of EM and DM devices, as shown in Figure 1. In an EM device, it does not conduct when VGS (gate-to-source voltage) is 0V. It starts conducting once the VGS reaches the gate-to-source threshold voltage, VGS(th). In contrast, the channel of a DM device is fully conductive at VGS=0V.

For EM devices, the drain current (ID) increases when VGS > VGS(th). On the other hand, for DM devices, the current increases when VGS > 0V. EM devices operate as open switches at VGS=0V, while DM devices operate as closed switches at VGS=0V.

In certain applications, enhancement-mode (EM) devices cannot replace depletion-mode (DM) devices because EM devices cut off at zero gate-to-source voltage (VGS). Furthermore, in some applications involving depletion-mode MOSFETs, there is no need for a gate drive circuit as the gate is biased from the application circuit itself. By utilizing the linear operation capability of depletion-mode MOSFETs, overall system costs can be reduced while complexity is reduced and reliability is improved.

Depletion-mode MOSFET Products

Littelfuse offers depletion-mode power MOSFETs based on the structure of vertical double-diffused MOSFETs (DMOSFETs). All these devices can operate in the linear mode, thanks to the extended forward bias safe operating area (FBSOA), resulting in higher reliability in terminal applications. Littelfuse’s depletion-mode MOSFETs include the Depletion D, Depletion D2, and Depletion CPC product series, providing a diverse range of depletion-mode (DM) products as illustrated in Figure 2.

Unlike EM devices, DM devices are not typically used in high-frequency applications. Generally, EM devices cannot operate in the linear mode except for linear MOSFETs [1]. However, all D-series and D2-series DM devices have an extended FBSOA, enabling them to operate in the linear mode. Currently, high-voltage depletion-mode MOSFET products with a rated voltage of 2500V are being developed. These depletion-mode MOSFET devices are required for applications such as high-voltage (HV) test equipment, power supplies, ramp signal generators, insulation resistance test equipment, or auxiliary power in high-voltage transmission systems. Figure 3 illustrates Littelfuse’s leadership position in the market for depletion-mode MOSFETs.

Applications of depletion-mode power MOSFETs

Here are some unique applications where depletion-mode MOSFET products are well-suited [3].

Start-up circuits in Switched-Mode Power Supplies (SMPS)

Traditional start-up circuit methods in SMPS involve power resistors and Zener diodes. In this approach, the power resistors continue to consume power even after the start-up phase, leading to excessive heat generation on the PCB, lower efficiency, and limitations in the input operating voltage range of the SMPS. An alternative approach using depletion-mode MOSFETs can be employed, as shown in Figure 4. Depletion-mode MOSFETs provide the initial current required by the PWM IC for start-up operation. After the start-up phase, an auxiliary winding generates the power required by the PWM IC. During normal operation, depletion-mode MOSFETs consume minimal power due to their low static current. The key advantage of this method is that the power dissipation after the start-up sequence operation theoretically approaches zero, resulting in improved overall efficiency. Additionally, it occupies a smaller PCB footprint and allows for a wide range of DC input voltages, which is crucial for many applications such as solar inverters.

Surge Protection in Linear Voltage Regulators

Linear voltage regulators provide power to small analog circuits, CMOS ICs, or any other low-current loads, with the input voltage Vin directly derived from the bus voltage. This can experience significant voltage variations, including voltage spikes due to application environments. As shown in Figure 5, depletion-mode MOSFETs can be utilized to implement surge protection in linear voltage regulator circuits. This MOSFET is configured with a source follower connection. The voltage on the source will follow the voltage changes on the gate. The conduction of the depletion-mode MOSFET depends solely on the gate voltage and is independent of the drain voltage. This configuration is employed to reduce voltage transients until the device’s rated voltage tolerance VDS is reached. The advantages of the depletion-mode MOSFET-based solution include a wide range of DC operating voltage Vin and minimal power consumption achieved by the low static current of the MOSFET. This protection feature can be used in communication applications to minimize the transient effects caused by surges. It can also be utilized in automotive and aerospace electronic applications to reduce transients caused by inductive loads.

Constant Current Source

Depletion-mode MOSFETs can be used to implement a constant current source, as shown in Figure 6. It provides a constant current to the load based on the resistor value (R) and the gate cutoff voltage (VGS(off)). Hence, the current (ID) remains independent of the voltage (Vin). The current is equal to ID = VGS(off) / R. Such a current source can be used in LED array drivers, trickle charging circuits to maintain battery level in monitoring systems, or for charging capacitors in a constant current manner.

Figure 6: Constant Current Source using Depletion-mode MOSFET

High-Voltage Ramp Signal Generator

Applications like automatic test equipment require high-voltage ramps that maintain a linear relationship between output voltage and time. Depletion-mode MOSFETs can be configured to design high-voltage ramp generators, as shown in Figure 7. A constant current source charges capacitor C through resistor R1, generating a voltage ramp (Vout) across the capacitor. The linear MOSFET can be controlled with a control signal to reset the ramp voltage, and the capacitor can be discharged to zero through resistor R2. Resistor R2 is used to limit the discharge current of the linear MOSFET, ensuring its operation within the Safe Operating Area (SOA) rating.

Figure 7: High-Voltage Ramp Generator using Depletion-mode MOSFET

High-Voltage Protection Circuit

Depletion-mode MOSFETs can be used for protection in measurement instruments to prevent damage caused by accidental connection to high voltage (Vmeas) by the measurement probe (Figure 8). In this case, a back-to-back configuration of MOSFETs S1 and S2 is used to protect the instrument by limiting the current. This provides protection against both positive and negative voltages on the probe. Such a circuit can be used in desktop or handheld instruments.

Figure 8: High-Voltage Protection Circuit using Depletion-mode MOSFET

Solid-State Relays

Depletion-mode MOSFETs perform well in replacing mechanical relays with solid-state relays (SSRs) for load switching, as shown in Figure 9. The primary advantages of SSRs are their immunity to magnetic fields, higher reliability due to the absence of mechanical contacts, and space-saving on PCBs. Solid-state relays find extensive use in applications such as medical equipment, industrial automation, measurement and test equipment, and consumer electronics.