--- title: "BJT switching characteristics and base drive" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:bjt-switching-gate-signal" source: https://www.bctengineer.com/q/semester_7/power_electronics/bjt-switching-gate-signal site: BCT Engineering --- # BJT switching characteristics and base drive Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2080 Baishakh, 8 marks - Video: [Switching Characteristics of Power BJT](https://www.youtube.com/watch?v=wHvF-yGOVos) — Vuluchala Nirmala ee_tech vignan wisdom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/bjt-switching-gate-signal - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain Switching Characteristics of BJT. Explain a gate signal Generating signal for BJT. ## Short answer BJT switching is characterized by turn-on time (delay plus rise) and turn-off time (storage plus fall) due to junction capacitances and stored base charge. A gate signal circuit uses a PWM source, opto-coupler for isolation, and a totem-pole driver that provides positive base current for fast turn-on and negative current to sweep out charge for fast turn-off. ## Answer 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 ```text iB ^ IB1 _____________ | | | |-----+ +----------- | |___ -IB2 iC ^ ___________ | /| |\ | / | | \ |-----' | | `---- ||| ||| 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 ```text +---------+ +-----------+ +---------+ +---------+ | 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which two intervals make up the turn-on time of a BJT? A. Storage time and fall time B. Delay time and rise time C. Delay time and storage time D. Rise time and fall time Answer: B. ton = td + tr, while toff = ts + tf. (Key point: Turn-on time) 2. What causes the storage time ts, and why is it usually the longest turn-off interval? A. Charging of the base-emitter capacitance to 0.7 V B. Excess carriers stored in the saturated base must be removed before iC can fall C. Collector voltage rising to Vcc D. Slow decay of the supply Answer: B. After the base drive is removed, stored excess carriers must be swept out first; this depends on the overdrive and takes the longest time. (Key point: Storage time) 3. What is the role of the lower transistor of the totem-pole driver in the base drive? A. Supply forward base current +IB1 for fast turn-on B. Pull a negative current -IB2 from the base to remove stored charge for fast turn-off C. Provide isolation from the power circuit D. Generate the PWM signal Answer: B. The upper transistor gives +IB1 at turn-on; the lower one gives -IB2 to remove stored charge at turn-off. (Key point: Totem-pole driver) 4. Why is an opto-coupler or pulse transformer used between the pulse generator and the BJT base driver? A. To amplify the base current B. To isolate the logic circuit from the power circuit whose emitter may float at high voltage C. To reduce the storage time D. To generate the PWM frequency Answer: B. They give electrical isolation between the low-voltage logic and the high-voltage power circuit. 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