Exploring The Features & Applications Of Littelfuse P-Channel MOSFETs

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Littelfuse p-channel power MOSFETs, although less renowned than their widely used n-channel counterparts, have traditionally served a limited range of applications. However, with the increasing demand for low-voltage (LV) applications, the scope of p-channel power MOSFETs has been expanded. The simplicity of applying Littelfuse p-channel solutions for high-side (HS) applications makes them more attractive for LV inverters (< 120 V) and non-isolated point-of-load solutions. The ease of driving an HS p-channel MOSFET without the need for a charge pump or additional voltage source results in simplified de­sign, space savings, reduced part count and cost efficiency.

This article makes a basic comparison of n-channel and p-channel power MOSFETs, introduces Littelfuse’s p-channel power MOSFETs, and then explores their target applications.

A comparative analysis of n-channel and p-channel power MOSFETs

To achieve the same on-state resistance R_DS(on) as the n-channel type MOSFET, the p-channel requires a 2 to 3-times larger die size. Therefore, in high-current applications, where low conduction losses are crucial, large die p-channel MOSFETs with very low R_DS(on) are suboptimal. While the larger chip size of the p-channel device offers improved thermal performance, it exhibits larger intrinsic capacitances and thus higher switching losses. This drawback significantly affects overall system cost, efficiency, and thermal management when the system operates at a high switching frequency.

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In low-frequency applications with significant conduction losses, a p-channel MOSFET should match the R_DS(on) of an n-channel MOSFET, requiring a larger chip area. Conversely, in high-frequency applications prioritizing switching losses, a p-channel MOSFET should align with the total gate charges of an n-channel counterpart, often having a similar chip size but a lower current rating. Therefore, choosing the right p-channel MOSFET demands careful consideration of device R_DS(on) and gate charge (Q_g) specifications along with the thermal performance.

Littelfuse p-channel power MOSFETs

Littelfuse offers a range of industrial qualified p-channel power MOSFETs with the highest voltage class, lowest R_DS(on) and Q_g, high avalanche energy rating, excellent switching performance and superior safe operating area (SOA) with best-in-class performance in both standard industrial and unique isolated packages. Littelfuse p-channel power MOSFETs retain the essential features of comparable n-channel power MOSFETs such as fast switching, efficient gate-voltage control, and excellent temperature stability.

Fig. 2 presents the key highlights of p-channel power MOSFETs offered by Littelfuse. Standard P and PolarP™ planar devices are available with voltage ratings from -100 V to -600 V and current ratings from -2 A to -170 A. PolarP™ offers optimized cell structure with low area-specific on-state resistance and improved switching performance, while Standard P benefits from a better SOA performance. Trench P utilizing a more dense trench gate cell structure offers very low R_DS(on), low gate charge, fast body diode and faster switching with device voltages ranging from -50 V to -200 V and currents from -10 A to -210 A. The latest addition to the portfolio is IXTY2P50PA (-500 V, -2 A, 4.2 Ω) which is Littelfuse’s first automotive grade p-channel power MOSFET for automotive applications.

Applications for Littelfuse p-channel power MOSFETs

N-channel MOSFETs are typically used in the power stage in typical half-bridge (HB) applications, . However, n-channel HS switches necessitate a bootstrap circuit to generate a gate voltage that is floating with reference to the source of the HS MOSFET or an isolated power supply to turn on as shown in Fig. 3a). Hence, the advantage of using n-channel devices comes at the cost of increased complexity in gate driver design leading to more design effort and greater space usage. Fig. 3 illustrates the contrast between circuits using complementary MOSFETs and those using n-channel MOSFETs. When a p-channel MOSFET is used as the HS switch in this configuration as depicted in Fig. 3b), it can greatly simplify the driver design. A charge pump that drives the HS switch could be removed and the p-channel MOSFET can be easily controlled by the MCU through a simple level shifter. This reduces design effort and part number count resulting in a cost-efficient design that utilizes space efficiently.

Reverse polarity protection

Reverse polarity protection is a safety measure within systems to prevent potential fire hazards and damages caused by a reverse connection of the power source. Fig. 4a) shows the reverse polarity protection implemented using a p-channel power MOSFET. When the battery is correctly connected, the intrinsic body diode conducts until the MOSFET channel is activated. In the event of a reverse connection of the battery, the body diode is reverse-biased, with the gate and source at the same potential, thereby turning off the p-channel MOSFET. A Zener diode clamps the gate voltage of the p-channel MOSFET, protecting it in case of excessively high voltage levels.

Load switch

Load switches connect or disconnect a voltage rail to a specific load offering a cost-effective and simple way for a system to efficiently manage power. Fig. 4b) demonstrates a circuit using a p-channel power MOSFET for a load switch. This circuit is driven by a logic enable (EN) signal controlling the p-channel load switch via a small-signal n-channel MOSFET Q1. When EN is low, Q1 is off, and the p-channel gate is pulled up to VBAT. Conversely, when EN is high, Q1 activates, grounding the p-channel gate, and turning on the load switch. If VBAT exceeds the p-channel MOSFET’s threshold voltage, it can turn on when EN is high, eliminating the need for an additional voltage source to bias the gate, which is necessary for n-channel MOSFETs. The series resistor is needed to limit the current and a Zener Diode to clamp the gate voltage to a maximum value.

DC-DC converters

Conclusion

With the development of modern LV applications, Littelfuse’s p-channel power MOSFETs prove their versatility in meeting the evolving needs of today’s power electronics. Employing Littelfuse’s comprehensive range of p-channel MOSFETs provides designers with advantages such as simplified, more reliable, and optimized circuit design in industrial and automotive applications. To achieve optimal performance for specific applications, designers must assess the trade-off between R_DS(on) and Q_g when selecting a p-channel power MOSFET.

Author: Sachin Shridhar Paradkar, Littelfuse