Which transistor type to use for charge shuttle, as part of an active charging circuit?
Hi All!
I've been looking at how to perform active battery balancing, and I'm currently struggling to understand which type of transistor to use to act as the switch for the "balancing capacitor".
Taking this example from a Texas Instruments paper as an example:
What would be a suitable transistor to use for the 2 way switches?
I've figure probably not an IGBT since their gate requires too much current, while a BJT would be too wasteful as well, so would that mean a MOSFET or JFET is the best choice?
2 answers
You need something with very low on-state voltage drop. That effectively means some type of FET. Those look resistive when on, as opposed to a voltage source like a BJT does.
You need to look carefully at the voltages when the switches are supposed to be off. The body diode of MOSFETs and the gate junction of JFETs may pose a challenge.
There are also totally different approaches to charge balancing. The one I've seen most is to have switchable shunts across each cell. Those are turned on for the cells above the median voltage during charging.
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I've figure probably not an IGBT since their gate requires too much current, while a BJT would be too wasteful as well, so would that mean a MOSFET or JFET is the best choice?
First, let's clarify that we are talking about a discrete solution, i.e. you buy some parts and assemble them in a circuit on a PCB. You're not asking about chip design.
Simple things first: JFETs are and were always small-signal devices. They are not really suitable for switching in power circuits. They are also slowly becoming unobtainium. So, nope to JFETs.
An IGBT has an insulated gate, so by the very name you can expect that probably an IGBT will need less gate current than a good power bipolar transistor needs base current, when they both conduct the same current.
The only current that flows into/out of the IGBT gate is the gate capacitance AC current. As you switch the IGBT, the gate-to-"channel" capacitance has to be continuously discharged and recharged. That's the gate current.
In that respect, IGBTs and MOSFETs are similar. Power IGBTs and MOSFETs have fairly large gates, and thus lots of gate capacitance. Think on the order of 1nF to 10nF. The gate switching square wave is shunted "to ground" via this capacitance.
BJTs have fairly small base capacitance. But they need base current just to conduct. So, while the switching transient current spikes from MOSFETs and IGBTs are not a thing on BJTs, the base will draw DC current any time the transistor is on. When the transistor is being switched, this current will be AC.
BJTs also have a problem that they are not very good switches in reverse direction. They break down with just a couple of volts of reverse base-emitter bias. Using BJTs as general purpose bidirectional switches in complicated because of that. You need to use two BJTs in series, and generate the base currents they need to conduct.
So, the simplest way to do the switching would be with a MOSFET since the battery voltage is presumed to be low. The gates would be driven via a gate isolation transformer. A single transformer with a pair of secondary windings, or with a central tap on the secondary, will happily drive an "ideal switch" back-to-back MOSFET pair.
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