Electronics > Beginners
Speed up BJT switching
npelov:
@Prithul0218
The inductor value is chosen with mix of calculations and simulations (more heavy on the simulations though). I actually came up with a value of 68uH for this frequency, but I only had 47uH for experimenting, so I just put lower limit on the on time to avoid saturating the inductor. In fewer words if the inductor is too low and the on pulse is long enough the inductor will saturate and the current will shoot trough the roof as the inductor at DC is effectively a short circuit. That's why I burned few transistors by messing up in firmware and having too much ON time.
You could use resistors instead of inductors and burn the power as heat (and also remove the Schottky diodes). If you calculate the current through the resistors and it'll allow 100% duty (on all the time) without burning anything. I decided I want to be fancy and use the energy from balancing for charging the whole pack. In my case that's not really necessary because at low currents (max average current through the inductors is about 400-500mA). That's 0.45A*4.2V=1.9W per cell max. 2W resistors would be fine - that's 4W max when balancing.
@Benta
R20 = 47k. Vr20=5V-0.7=4.3V. Ir20 =4.3/47k = 91uA * hfemin=400 = 36mA max drive for Q4 base. With 1k that's 10V min/1k = 10mA (3 times less). It should be fine. I could use 20k just to be on the safe side.
Hmm. I found so I decided to put a schottky on base-collector of Q4 (Cathode to base). It didn't work. It just adds some oscillation of the base voltage on turn off mostly and some on turn on. Graph attached. Ic still drops slowly
npelov:
Here is a graph (first attachment) of base voltage and current with no speedup:
Base voltage is yellow (x10), Base current is red x100.
My guess is: The first part where base voltage almost doesn't drop even though the base current dropped is the delay caused by the hard turn on. The second part is the base capacitance discharging. R2 is taking care of the second part.
Schottky across base-collector should take care of the first part, but it doesn't - see second graph. I didn't remove the other parts, just put very low/high values to negate them. I guess the inductor messes it up because technically there is no load on the collector at the start of the off pulse.
T3sl4co1l:
Note that you have maximum Miller effect in this configuration, which makes the large rectifier 1N5817 rather undesirable. A signal diode like BAT85 or BAT54 would be more suitable.
A PN diode can also be used, in which case you need to place it on a tap higher up the base divider:
You can use this method to set an arbitrary Vce(sat) value, useful for low-accuracy clamping applications.
Alternately, a PN diode is fine with a Darlington transistor,
But there is a caveat: the real circuit will oscillate. The left circuit simulates as stable, but the real circuit oscillates. The right circuit has been modified to reproduce the real oscillation. Unfortunately, the parasitics don't happen to be physically realistic: the load resistor R1 is a wirewound with about 50 times lower inductance than shown here, for example.
Probably the root cause of this discrepancy is, a real transistor has some delay, or higher order poles, whereas SPICE has no concept of delay*, and has a single-order model (lumped terminal capacitances, current gain goes to unity at fT).
*Except when transmission lines are explicitly used. (Built-in components like transistors do not use them.)
Anyway, for faster switching, you need to discharge the B-E junction. BJTs are voltage-controlled, not current. Instead of an open-collector driver (active pull-down, Q2), have an active pull-up phase as well. Then you can use the speed-up cap across the series resistor to do the business.
Also, consider MOSFETs. These have lower gain (= needs more drive voltage), but that's not such a big deal (e.g., use a bootstrap gate driver IC), and you don't have stored charge or Vce(sat) to worry about.
Finally, as for the general circuit and apparent purpose -- I don't see any way to control or limit current here, nor will the currents or voltages be balanced in any obvious way, with the values shown. The canonical way to build a balancer circuit is to use a dedicated regulator IC for each cell, sending excess charge current back into a common rail. Each regulator controls its own inductor current and cell voltage independently, giving maximum benefit, without much added cost (regulators are cheap, exploding cells are not).
Tim
Benta:
Schottky rectifiers are completely unsuited for BJT base clamping. Use Schottkys designed for switching as Teslacoil writes.
Zero999:
I underestimated the reduction in base current, due to the base-emitter resistor, earlier on, but I maintain it had nothing to do with the reduced turn-off time.
I've done a simulation with one circuit with the base-emitter resistor and the other with out. The base resistor values were selected to keep the base currents similar. Notice on the circuit with the base-emitter resistor, the sign of IB changes, when the transistor turns off and is why it turns off much faster, then the one without.
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