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P Channel and N Channel MOSFETs in Switched Mode Power Supply Applications

MOSFETs have been the chosen transistor technology in most Switched Mode Power Supplies (SMPS) since the mid 1980’s. MOSFETs are used as the main switching transistor as well as to improve efficiency when used as gated rectifiers. In order to choose the best switch for power application this blog provides a comparison of P Channel and N Channel enhancement mode MOSFETs.

As an acronym for marketers, MOSFET could stand for Most Optimal Solution for Energy Transfer. For engineers it stands for Metal Oxide Semiconductor, Field Effect Transistor.

Due to its lower on resistance and smaller size, the N Channel MOSFET exceeds the P Channel in number of product choices. In buck regulator applications, either P Channel or N Channel MOSFETs are used as the main switch based on several factors including gating voltage polarity, device size, and series resistance. Synchronous rectifier applications almost always use N Channel technology. This is mainly because the N Channel has the best RDS(on) of the two types and is turned on by a positive voltage on the gate.

MOSFETs are majority carrier devices whereas N Channel type MOSFETs have electrons flowing during conduction. P Channels use positive charges referred to as holes during conduction. Electrons have three times the mobility of holes. Although there isn’t a direct correlation, it takes approximately three times the die size for a P Channel to meet the equivalent N Channel in terms of RDS(on) . Hence die size of an N Channel is smaller.

N Channel MOSFETs turn on when a positive voltage of the appropriate threshold value is applied across the gate-to-source terminals. P Channel MOSFETs turn on by applying a given value of a negative gate-to-source voltage.

The gating of MOSFETs determines their application in SMPS converters. For example, low side switching that references ground favor N Channel MOSFETs especially in boost, SEPIC, forward, and isolated flyback converters. Low side switches are also being used to replace diodes as rectifiers in synchronous rectifier converter applications as well as for Power over Ethernet (PoE) input rectifiers [5]. P Channels are most often used for high side switches in buck regulators where the input voltage is lower than 15 VDC. Depending on the application, N Channel MOSFETs can also be used as buck regulator high side switches. For these applications, a bootstrap circuit or other form of high side drive is needed.

High-Side Drive IC Featuring a Level-Shifter

High-Side Drive IC Featuring a Level-Shifter

Gating a High Side N Channel MOSFET with a Bootstrap Circuit

Gating a High Side N Channel MOSFET with a Bootstrap Circuit

Polarity determines the graphic symbol of a MOSFET. The differences are in the orientation of the body diode and the arrow symbol with respect to the terminals.

MOSFET Schematics for P Channel and N Channel MOSFETs.
Note the Orientation of the body diode and arrows with respect to the Drain (D) and Source (S) terminals.

MOSFET Schematics for P Channel and N Channel MOSFETs. Note the Orientation of the body diode and arrows with respect to the Drain (D) and Source (S) terminals.

Polarity determines the MOSFET’s operating characteristics. The currents and voltages that are positive for an N Channel Device are negative for the P Channel device.

MOSFET First Quadrant Characteristics

MOSFET First Quadrant Characteristics

MOSFETs are “fully ON” in the ohmic region where a sufficient voltage is applied to the gate-to-source terminals. In the comparison Figure, VGS for the N Channel ohmic region is 7 volts whereas the value is -4.5 volts for the P Channel.

The slope of the curves in the ohmic gets steeper as gate voltage increases thus indicating the device is becoming more conductive. MOSFET exhibit lower RDS(on) with higher applied gate voltages. In some applications the MOSFET is gate with a voltage that provides a favorable RDS(on) . This additional gate voltage causes power loss due to the ½ C x Vgs x Vgs x f where both gate charge and switching frequency play a role in determining the final operating point and choice of MOSFET technology.

In addition to operation in the first quadrant MOSFETs can be operated in the 3rd quadrant. The parasitic body diode conducts when no gate-to-source voltage is applied. When a gate voltage is absent, the current into the drain resembles a typical diode curve.

Typical Characteristics for  Third Quadrant Operation for an Ungated, N Channel MOSFET

Typical Characteristics for Third Quadrant Operation for an Ungated, N Channel MOSFET

When a gate voltage is applied it creates a nonlinear curve depending upon the value of VGS. The Device When VGS Exceeds 10 Volts the N Channel MOSFET operates fully in the third quadrant ohmic Region. However, the diode voltage clamps at various levels of drain current for gate voltages below 10 volts. This bend that is observed in the nonlinear curves is transition point between diode and ohmic regions.

Typical Characteristics for Third Quadrant Operation for and N Channel MOSFET with an Applied Gate Voltage

Typical Characteristics for Third Quadrant Operation for and N Channel MOSFET with an Applied Gate Voltage

A summary table provides a comparison N Channel and P Channel MOSFETs.

Parameter Orientation for the N Channel Orientation for the P Channel Gate-to-source voltage VGS Required to turn the device on Positive Negative Drain Current Positive into the Drain Negative into the Drain Arrow in Device Symbol Points towards the gate Points away from the gate Body Diode Orientation Source to Drain (anode to cathode) Drain to Source (anode to cathode) Fairchild portfolio Maximum VDS rating 1000 V 500 V

References

  1. Website: Fairchild P Channel MOSFETs > 250V
  2. Website: Fairchild N Channel MOSFETs > 250V
  3. Website search results for MOSFETs
  4. “Design and Application Guide of Bootstrap Circuit for High-Voltage Gate-Drive IC.”. Fairchild Application Note AN-6076, Rev 1.4, 12/18/2014
  5. “FDMQ8203 Greenbridge Series of High-Efficiency Rectifiers,” Fairchild data sheet, Rev C1, 2011 data sheet
  6. “A New PSPICE Subcircuit for the Power MOSFET Featuring Global Temperature Options,” Fairchild Application Note AN-7510

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