A bipolar junction transistor (BJT) can perform as both a small-signal amplifier or a swap. Although you don’t see many discrete BJT amplifiers on circuit boards nowadays—it’s vastly extra handy and efficient to make use of an operational amplifier—it’s nonetheless widespread to come across BJTs linked as switches.
BJT switches are sometimes used to dam or ship present to masses like brushed DC motors, lamps, or solenoids. Additionally they generally seem in higher-frequency switching functions similar to switch-mode regulators or Class D amplifiers. Determine 1 exhibits two widespread functions for a BJT swap: high-intensity LED illumination (left) and relay management (proper). Each switches are actuated by a general-purpose enter/output pin on a microcontroller.
Determine 1. Two examples of BJTs functioning as switches.
When designing BJT swap circuits, our focus tends to be on the currents and voltages we have to correctly management the transistor and drive the load. Nonetheless, it’s additionally vital to think about energy dissipation, particularly in battery-powered or high-ambient-temperature functions. If we don’t, the BJT’s losses could enhance part temperatures to the purpose of impaired efficiency and even thermal failure. On the very least, energy dissipation will cut back the swap’s effectivity.
On this article, we’ll concern ourselves with two main kinds of energy dissipation: conduction loss and transition loss.
BJT Conduction Loss
As a swap, a BJT all the time operates in certainly one of two modes:
- Absolutely off. No load present can move and energy dissipation is basically zero.
- Absolutely on. Load present flows freely and energy dissipation is low however non-zero.
Within the on-state, the load present flows from the BJT’s collector to its emitter. A base-to-emitter present can also be required to make collector-to-emitter conduction potential. The whole energy dissipation of those two present paths is called the conduction loss (PC). We are able to calculate it utilizing the next formulation:
$$P_C~=~left(V_{BE}~occasions~ I_Bright)~+~left(V_{CE}~occasions~ I_Cright)$$
the place:
VBE is the voltage throughout the base-to-emitter junction
VCE is the voltage throughout the collector-to-emitter junction
IB is the bottom present
IC is the collector present.
Throughout conduction, VBE is often round 700 mV. When the BJT is in saturation, which is the popular mode for switching functions, VCE is round 200 mV. We are able to receive a tough estimate of the conduction loss by assuming these fastened values, then figuring out base and collector present by way of customary circuit evaluation methods.
Utilizing LTspice to Estimate Conduction Loss
SPICE simulations present one other, extra correct method of estimating conduction loss. Think about the LTspice circuit in Determine 2, for instance. Q1 of this simulated bipolar junction transistor is managed by a 3.3 V digital sign and switches present to a 50 Ω load.
Determine 2. A bipolar junction transistor modeled in LTspice.
Determine 3 exhibits the base-to-emitter and collector-to-emitter voltages that consequence once we run the simulation.
Determine 3. Base-to-emitter voltage and collector-to-emitter voltage throughout the energetic portion of the switching cycle.
The LTspice plot exhibits a VCE of 208.5 mV, which could be very near the 200 mV worth we assumed within the previous part. In contrast, VBE is considerably larger than we assumed—934 mV as a substitute of the anticipated 700 mV.
We may insert these new values into our circuit-analysis calculations and generate a brand new estimate for conduction loss, however it’s a lot simpler to let LTspice do the maths for us. Simply maintain down the Alt key—or the Command key, should you’re utilizing a Mac—and click on on the transistor; LTspice will generate a plot just like the one in Determine 4.
Determine 4. Transistor energy consumption calculated and plotted by LTspice.
The outcomes point out that this BJT swap will dissipate a constant 56 mW of energy throughout the energetic section of the switching cycle.
BJT Transition Loss
These ominous spikes within the power-consumption plot above recommend that conduction loss isn’t the one kind of energy dissipation that we have to talk about. Determine 5 exhibits what occurs if we zoom in on a type of spikes.
Determine 5. BJT energy dissipation throughout the transition from the non-conducting cutoff state to the saturated conducting state.
These spikes happen as a result of a BJT can’t change instantaneously from a non-conducting state to a totally conducting state. Through the transition, vital collector present is flowing, and the collector-to-emitter voltage hasn’t but settled into its low saturation degree. Energy dissipation is due to this fact comparatively excessive.
You may see these present–voltage dynamics in Determine 6. The orange and pink curves plot collector voltage and collector present, respectively; the inexperienced curve plots energy dissipation.
Determine 6. Collector voltage, collector present, and whole BJT energy dissipation throughout the transition from the switch-off state to the switch-on state.
There’s no simple option to precisely calculate transition losses. A number of variables are concerned, and the BJT’s currents and voltages change in moderately complicated methods. I recommend utilizing simulations.
Let’s take a look at an instance. Beginning with the plot above, I can maintain down the Ctrl key and click on on the waveform label to carry out integration (Determine 7). The world beneath the facility curve represents vitality loss, and this vitality will be added up and divided by time to provide the common energy dissipation as a result of BJT transition.
Determine 7. Integrating an instantaneous-power waveform with LTspice.
This means that every transition causes about 1.35 μJ of vitality loss. Let’s say we’re switching at 500 Hz, or 5 hundred cycles per second, which corresponds to at least one thousand transitions per second. The whole vitality loss per second can be 1.35 μJ × 1000 = 1.35 mJ. The typical energy dissipation as a result of transitions is due to this fact 1.35 mW.
Even in conditions the place you don’t want a numerical estimate, try to be aware of the next two parameters:
- Switching frequency. The next switching frequency means extra transitions per second and due to this fact larger time-averaged loss.
- Rise/fall occasions. Longer rise or fall occasions create extra vitality loss per transition.
Each of those elements strongly affect transition losses. For instance, Determine 8 demonstrates that rising the management sign’s rise time from 10 μs (the worth used for the above simulation) to 100 μs raises the vitality loss from 1.35 μJ to 13.7 μJ.
Determine 8. Slower transitions between on and off states result in extra vitality loss.
Wrapping Up
As we noticed on this article, SPICE simulation is a invaluable instrument for analyzing and predicting BJT switching losses. Understanding these sources of energy dissipation will help designers to optimize their circuits and make sure that elements aren’t burdened or broken by excessively excessive temperatures.
All photographs used courtesy of Robert Keim