MOSFET Selection for Buck Converter

Buck converter can be synchronous or non-synchronous type. The former uses a MOSFET (or BJT, IGBT) in place of the buck diode. Basically, there are two switches in synchronous type. On the other hand, the latter uses a switch and a diode combination. This is simpler and cheaper than the former since there is only a single MOSFET to control. The cost of MOSFET could be higher than diode in high power applications. This topic will guide you in MOSFET selection for buck converter use, either synchronous or non-synchronous type. For detailed explanation about buck converter design, read Buck Converter Design Tutorials.

Parameters to Watch Out in MOSFET Selection for Buck Converter

The important parameters to look for are RMS current, voltage, power dissipations and thermals. Power dissipation and thermals are interrelated to each other and depends on several factors like drain to source on state resistance (R_DSon), total gate charge (Qgtotal), output capacitance (C_OSS), rise time and fall time and switching frequency.

We used RMS current over DC current because RMS is higher than DC in a trapezoidal waveform of the switch current in a buck converter. The consideration of the current is simple. The computed or actual current must not exceed the device rating. Voltage is also an important parameter to consider in MOSFET selection for buck converter applications. This could be a drain to source voltage, gate to source voltage or the gate to source threshold voltage.

8 Items Must Do in MOSFET Selection for Buck Converter

1. Do not Over Current the Drain

By knowing the actual RMS current, you can select the MOSFET current rating. For a non-synchronous buck converter, the MOSFET RMS current (Q1 in the illustration) could be computed as

IRMS Q1 = sqrt (duty cycle) x [load current – (di/2) + (sqrt (3) x di / 3)]

di is can be solved using this equation:

di = Vout x ( 1 – duty cycle ) /( switching frequency x L1 )

duty cycle is simply

duty cycle = Vout / Vin

Example:

Vin = 20V, Vout = 10V, switching frequency = 100 kHz, L1 = 10 uH, load current = 10A

Solution:

duty cycle = Vout / Vin = 10V / 20V = 0.5

di = 10V x ( 1- 0.5 ) / ( 100 kHz x 10 uH ) = 5A

IRMS = sqrt (0.5) x [ 10A – 5/2 + sqrt (3) x 5/3 ] = 7.35A

Select a Q1 MOSFET with RMS current rating much higher than 7.35A.

For the synchronous buck, the rms current of Q2 MOSFET is

IRMS Q2 = sqrt ( 1-duty cycle ) x ( load current – di / 2 + sqrt ( 3 ) x di / 3 )

= sqrt(1-0.5) x (10A – 5/2 + sqrt(3) x 5/3) = 7.35A

Select a Q1 MOSFET with RMS current rating much higher than 7.35A. Q1 and Q2 current do not equate all the time. It just happens the duty cycle is 50%.

Good drain current rating to start with to MOSFET selection for buck converter is at least twice the computed maximum drain current.

2. Do not Apply Excessive Voltage to Drain to Source

MOSFET is rated in terms of drain to source voltage. The maximum drain to source voltage of Q1 MOSFET is ideally equal to the level of the input voltage. This will happen when Q1 is off while Q2 is on. Thus, select Q1 MOSFET with drain to source voltage much higher than the input voltage for more margin.

On the other hand, the maximum drain to source voltage of the Q2 MOSFET is happening when Q1 is on and Q2 is off. This is still ideally equal to the input voltage. Thus, select Q2 MOSFET with drain to source voltage much higher than the input voltage for more margin.

Good value to start for the drain voltage rating is at least twice the computed maximum drain voltage.

3. Do not Apply Excessive Voltage to Gate to Source

The gate to source of a MOSFET has a limit as specified in the datasheet. For instance, a maximum limit of 20V, then the applied voltage to the gate to source must be lower than 20V.

A good level to start with is to limit the applied gate to source voltage to just 70% of the MOSFET gate to source voltage capability. However, this is not the lone consideration. Continue reading item 4.

4. Ensure to Over Come the Gate to Source Threshold Voltage

For a MOSFET to turn on, the gate to source threshold voltage must be satisfied. For instance, the datasheet specifies a maximum gate to source threshold voltage of 4.5V, the actual voltage provided must be higher than this. However, the value cannot be exaggerated as the MOSFET gate to source has voltage limitation as described in item 3. The typical applied gate voltage must be

Gate to source threshold voltage < Gate Applied Voltage < Gate to source voltage limit

The applied gate voltage is also critical in terms of power losses because a higher gate voltage will result in higher switching loss. Read How to Compute MOSFET Switching Losses.

On the other hand, insufficient applied gate voltage will result in not obtaining a hard saturation state of the MOSFET channel and increase conduction loss. Read How to Compute MOSFET Conduction Losses.

5. Select a MOSFET with Low RDSon

R_DSon or the drain to source on state resistance is the main contributor of a conduction loss. The square of it is directly proportional to the conduction loss. Thus, the lower its value, the better in terms of efficiency.

6. Select a MOSFET with Low in Dynamic Parameters

Dynamic parameters are total gate charge, output capacitance, rise and fall times. They are the factors of switching losses. Read How to Compute MOSFET Switching Losses for more details. These parameters must be low to increase efficiency.

7. Select a MOSFET with High Maximum Junction Temperature

A higher junction temperature rating means a MOSFET can handle more heat. This is advantageous because power electronics typically generate heat.

8. A MOSFET Must Have Low Thermal Resistance

The last but not the least tip to MOSFET Selection for Buck Converter is the thermal resistance value.

Thermal resistance could be junction to ambient or junction to case. For power electronics, the latter has mostly been used. The preferable value is small as it will result to a higher power handling capability of the MOSFET.

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