The Advantages of Using a MOSFET Over a BJT in a Switching Converter


In the past years power supply designers and engineers are continuously searching for innovation. Long time ago what is available is only the linear type power supply. a linear power supply will consume a lot of power in order to provide regulation. This huge power consumption sacrifices the efficiency of the power supply. Try to imagine if you are only consuming 10 watts for your devices but the power concessionaire charged you with 15 Watts. This is ridiculous. Where did the 5 watts come from? This is the internal consumption of the power supply. A power supply role is to convert a particular input voltage and current into a form usable by the load and it should not consume the power itself. Yes, there is no perfect system and superconductors are still impossible to implement for some reason so a power supply will always has some losses. However, it should be very small such that the efficiency is not less than 90%.

With the drawback of a linear type power supply, power engineers shifted their attention to switching converters. Unlike the linear type which the regulating element is always in the conduction mode, in switching converter on the other hand is operating in the cut-off and the saturation of the switching element. At cut-off, there is no current therefore the power is zero. On the other hand, in saturation, the current is high but the voltage is zero ideally then again the power is zero.

The two popular switching elements are MOSFET and BJT. In early days, the latter is the widely used because it was discovered first. When the MOSFET become popular, the direction of the power supply design is towards it.

Advantages of MOSFET

1. The obvious advantage of a MOSFET is its very low drain-source on state voltage drop because of a very low drain-source on state resistance (RDSON). For a BJT, the collector-emitter saturation voltage may vary with the level of the base current and the collector current or simply the load.

2. MOSFET is voltage dependent which makes it easy to drive and set the operation into hard saturation. The gate-source voltage will not be affected by the level of the drain current and thus the possibility of operating at linear mode is very remote. While for BJT; it is a current controlled device that means the base current should be significant enough to drive the device into hard saturation. In the event of heavy loading, the degree of saturation of the BJT may be changed or worst case enter linear mode. By the way, saturation level is a bit wide. For example, generic BJT has a saturation level of 0.5 volts and below. Of course we don’t want to operate in the 0.5 volts region because this will correspond to a higher power loss. The idea is to drive the BJT into hard saturation so that the power loss is ideally zero.

3. MOSFET has lower input power loss than a BJT. The equivalent power loss of the BJT is the sum of the input capacitance and the VBE loss. The former is just a small portion compared to the latter. The power loss due to VBE is the product of the base current and the VBE voltage. We know that a transistor VBE is typically 0.7 volts. At higher base current, this 0.7 volts will give higher loss. For the MOSFET, the input loss is due to the total gate charge, switching frequency and the drive voltage. Nowadays total gate charge of MOSFETs is already very small so the loss as well.

4. MOSFET is reliable for high current application. You can assured a steady hard saturation even at short circuit operation. For a BJT however, the collector-emitter voltage drop will increase when the collector current increase and it will most likely to operate in the linear region during short circuit.