Power transistor switching and base drive

A power transistor does not switch instantly. Its switching characteristics describe the delay and transition times of collector current when base drive is applied and removed; a good base drive circuit shortens these times.

Switching characteristics

Switching characteristics: when a base pulse is applied and removed, the collector current does not follow it at once, because of the junction capacitances and the stored charge in the base.

 iB  ^  IB1 _____________
     |     |             |
     |-----+             +-----------
     |                    |___ -IB2
 iC  ^          ___________
     |        /|           |\
     |      /  |           |  \
     |-----'   |           |   `----
     |<td>|<tr>|           |<ts>|<tf>|
           ton                 toff

• Delay time $t_d$: time for the input capacitance of the base-emitter junction to charge to about 0.7 V; $i_C$ rises from 0 to 10% of $I_{CS}$.
• Rise time $t_r$: $i_C$ rises from 10% to 90%. $t_{on} = t_d + t_r$.
• Storage time $t_s$: after the base drive is removed or reversed, the excess carriers stored in the base (saturation) must be removed before $i_C$ can fall. It is the longest interval and depends on the overdrive.
• Fall time $t_f$: $i_C$ falls from 90% to 10%. $t_{off} = t_s + t_f$.

Switching losses occur during $t_r$ and $t_f$, when both voltage and current are large. They rise with switching frequency.

Turn-on and turn-off control by base drive

Base drive (gate signal) circuit: the base drive must (i) give a high base current quickly at turn-on, (ii) keep just enough base current to hold saturation during conduction, and (iii) pull out the stored charge with a negative base current at turn-off.

               C1
          +----||----+
   vin    |          |    R2
   o------+--/\/\/---+--/\/\/--> B
             R1                 (power
                                 BJT)
   GND -------------------------> E

1. Turn-on control: at the rising edge of $v_{in}$ (high level $V_1$), capacitor $C_1$ acts as a short across $R_1$, so a large initial base current $I_{B1} \approx \frac{V_1 - V_{BE}}{R_2}$ flows and the BJT turns ON fast (short $t_d$ and $t_r$).
2. Conduction: $C_1$ charges and the base current settles to $I_B = \frac{V_1 - V_{BE}}{R_1 + R_2}$, which is just enough to keep the transistor in (near) saturation, so the storage time stays small.
3. Turn-off control: when $v_{in}$ goes to zero or negative, the charged $C_1$ forces a reverse base current $-I_{B2}$, which sweeps out the stored base charge quickly and reduces $t_s$ and $t_f$.

Other refinements: a Baker (anti-saturation) clamp diode from base to collector keeps the BJT just out of hard saturation; proportional base drive makes $I_B$ proportional to $I_C$; and an opto-coupler or pulse transformer isolates the logic circuit from the power circuit.