BJT switching characteristics and base drive

The switching characteristics of a BJT show how its collector current and voltage change with time when the base drive is switched ON and OFF. Because of junction capacitances and charge stored in the base, the transistor needs a finite turn-on time and turn-off time.

Switching characteristics

 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.

Base (gate) signal generating circuit

 +---------+   +-----------+   +---------+    +---------+
 | 555 or  |-->| opto-     |-->| totem   |--->| base of |
 | uC PWM  |   | coupler   |   | pole    | Rb | power   |
 | (logic) |   | isolation |   | driver  |    | BJT     |
 +---------+   +-----------+   +---------+    +---------+
                                 +V / -V
                                 supply

1. A PWM or pulse generator (555 timer, comparator or microcontroller) sets the ON time and frequency.
2. An opto-coupler (or pulse transformer) isolates the logic circuit from the power circuit, whose emitter may float at high voltage.
3. A totem-pole (push-pull) driver with a $+V$ and $-V$ supply amplifies the signal: the upper transistor supplies forward base current $+I_{B1}$ to turn ON quickly; the lower transistor pulls a negative current $-I_{B2}$ from the base to remove stored charge and turn OFF quickly.
4. A speed-up capacitor across $R_b$ gives a high initial base current and a negative spike at turn-off; a Baker clamp diode avoids deep saturation, reducing storage time.