BJT is not a preferred choice for switching converters and power supplies especially when operating at high frequencies and high power. Here are the 6 reasons why BJT is not suitable for switch mode power supply.
(1) Long turn on and fall times
(2) Very long storage time
(3) High power dissipation on collector-emitter
(4) Significant power dissipation on base-emitter
(5) Current controlled, complicated to drive
(6) Easy to go thermal runaway when connected in parallel
1. High Turn On and Fall Times
The first of the 6 reasons why BJT is not suitable for switch mode power supply and switching converter is high turn on and fall times.
Power BJT has turn on and fall times in micro second range as compared to MOSFET which is in the nano second range.
BJT turn on time is the delay time plus rise time. Rise time is generally defined as the time the collector current to reach 90% of its settling value. This is an indicator of how fast a BJT enters to hard saturation. Fall time on the other hand is the time to take for the collector current to drop to 90%. This is an indication on how fast the BJT to exit saturation and go to cut-off.
Why these matter?
A higher turn on time means that the BJT is taking too long in the active region before going to full saturation. This time, there is a significant voltage in the collector-emitter junction while the collector current is rising as well. This will result to a high-power dissipation in the collector-emitter junction. We don’t want this as the efficiency will suffer and may result to reliability issue, such as thermal issue and shorten the life of the BJT.
The same explanation can be used to a higher fall time. There is a huge power dissipation in the collector-emitter junction once there is a significant time when the collector voltage and current overlaps during turn off.
Below table is comparing a BJT and MOSFET dynamic parameters. The left side is a BJT with a VCEo of 400V and 12A collector current. The right side is a MOSFET with drain voltage of 400A and 10.5A drain current. They are both TO-220 package. Both parts are from ON semiconductor. As you can see, the turn on and fall times of BJT is in microsecond range while the MOSFET turn on and turn off times are in nanosecond range.
The power dissipation associated by the turn on and fall times of a BJT is dynamic in nature. It is frequency dependent, which means this power dissipation is high when the switching frequency is high.
To put numbers to the argument, let us use below equation to quantify the switching loss. This is a good estimation to quantify the turn on and fall times loss of BJT.
Switching loss (BJT) = 0.5 x (ton + tfall) x Ic x Vc x Fsw
Where;
- ton is the turn on time of the BJT
- tfall is the fall time of the BJT
- Ic is the collector current (assume it to be 5A for this example)
- Vc is the collector voltage (assume this to be 100V for this example)
- Fsw is the switching frequency (assume this to be 100kHz for this example)
Switching loss (BJT) = 0.5 x (1.1usec + 0.7usec) x 5A x 100V x 100kHz = 45 watts
The above equation is also applicable to MOSFET with little change in variable names.
Switching loss, (MOSFET) = 0.5 x (trise + tfall) x Id x Vdrain x Fsw
Where;
- trise is the rise time of the MOSFET
- tfall is the fall time of the MOSFET
- Id is the drain current (assume it to be 5A for this example)
- Vd is the drain voltage (assume this to be 100V for this example)
- Fsw is the switching frequency (assume this to be 100kHz for this example)
Switching loss, (MOSFET) = 0.5 x (190nsec + 170nsec) x 5A x 100V x 100kHz = 9 watts
The numbers don’t lie. BJT switching loss due to turn on and fall times is very big!
2. Very Long Storage Time
BJT has a storage time. It is the time difference from the fall of the base current to the rise of the collector current. Storage time is inherent in BJT due to the minority charge carrier. When the BJT is turned off by removing the base current, the collector current will continue to conduct until the storage time expired. This is very problematic. It can cause switching loss and most of all impaired the operation especially when running at high switching frequency.
To demonstrate the impact of BJT storage time, let us consider a simple simulation using LTspice below.
V1 is a pulse source with a 20kHz switching frequency (50usec period) and a turn on time of 10usec. As you can see, the base current in blue is already zero but the collector current in green is still at 100% until around 1.5usec.
Let us make the switching frequency 100kHz (10usec period) and a duty cycle of 80% (8usec on time). Look at what happens. When the base current is cut-off (the BJT is commanded off), the collector current continues and cannot go to zero. In short, the switching action is gone. This will result to continuous turn on of the BJT. If this BJT is used in a half bridge together with the same BJT, shoot through will occur that will blow the BJT right away.
3. High Power Dissipation on Collector-Emitter Junction
A BJT has a collector-emitter junction. In order to minimize the power dissipation, this junction must be driven into hard saturation. However, the collector-emitter saturation voltage (VCEsat) of a BJT is significantly high by nature. At high collector current, this collector-emitter voltage will result to a very high-power loss (conduction loss). On the other hand, MOSFET has plenty of options with very low RDSon.
At low power application, BJT conduction loss due to its VCEsat is can be comparable to MOSFET’s conduction loss. However, there is a huge difference once operating power increases.
4. Significant power dissipation on base-emitter
In order to obtain a very low voltage drop across the collector-emitter junction of a BJT, it must be driven into hard saturation. To do so, the base current should be high enough. The base-emitter junction is a diode. A diode has a voltage drop that is either 0.3V or 0.7V for Germanium or Silicon material. In fact, if you look into Mouser or Digikey, the base-emitter voltage (saturation VBE) can even reach closer to 1V. The base current multiplied by VBE saturation voltage will result to a power dissipation.
5. Current controlled, complicated to drive
In order to correctly bias a BJT, there are two conditions to satisfy. First is to ensure that the VBE requirement is met. Second, ensure that the base current is enough to drive the BJT into a particular operating mode. That is why it is called a current-controlled device. The base current is dependent to the biasing resistor and base circuit configurations. This must be calculated carefully.
On the other hand, a MOSFET is straight forward to turn on. It needs only a voltage applied to its gate and source leads. There are no current constraints and no biasing resistor required.
6. Easy to go thermal runaway when connected in parallel
The base-emitter junction of a BJT is a diode. The voltage drop of a diode will decrease when the temperature is high.
When there are two BJTs connected in parallel, one of them may turn on first. This is reality, even though the BJTs are identical. The first BJT to turn on will heat up first, heating up as well the diode in the VBE. The voltage drop will decrease. This will result to a higher base current and will drive the BJT into hard saturation even more. As a result, the BJT that turns on late can no longer catch up in sharing the load. The heated BJT will fail and the entire load will be shouldered by the remaining BJT and it will fail immediately also.
Summary
- BJT is inferior to MOSFETs in switching applications such as in switching mode power supply (SMPS) and switching converter.
- BJT is still applicable to use in switching at low frequency. At low frequencies (less than 10kHz), the effect of the higher turn on and fall times of the BJT is can be minimal.
- At low power applications, the impact of the VCEsat to the conduction loss is can be managed.
- In using a BJT in switching applications (such as in SMPS), ensure that the storage time is lower than the PWM turn off time with a margin. This is to ensure that the BJT can turn off.
- In order to attain a very low VCEsat, BJT must be driven into hard saturation. To do so, the base current must be high enough and this result to power dissipation in the base-emitter junction.