Billy Long
For at least the last 20 years, MOSFETs have been the switch of choice for many switched-mode power-supply designs. Because of their higher switching speed and easier drive characteristics, MOSFETs have replaced bipolar junction transistors (BJTs) in many applications and power levels. However, for some applications – such as flyback-based low-power AC/DC chargers – BJTs have some distinct advantages over MOSFETs.
Because of their different device structure, high-voltage BJTs are less costly to make than high-voltage MOSFETs. For this reason, BJTs with voltage ratings up to or over 1kV are available for a lower price than the 600V or 650V MOSFETs often found in universal-input offline-flyback converters.
The advantage is immediately obvious. Because of the BJT's higher voltage rating, the leakage spike can be several hundred volts higher and yet still within the required switch derating. Depending on the magnitude of spike, it is often possible to remove the snubber completely without over-volting the switch.
Advantages:
Another area where you can take advantage of a BJT's high voltage rating is in applications with high-voltage or three-phase inputs. A standard European 230V three-phase input will have a peak line-to-line voltage of ~565Vdc. This peak voltage is often a rating requirement for equipment connected across a single phase, in case a fault in one phase of the load causes the neutral to be pulled to one of the line voltages. While many designers achieve the switch rating for this condition by using a large, expensive and higher-RDSON MOSFET or by cascading two lower-voltage MOSFETs in series to achieve the desired rating, using a single high-voltage BJT instead reduces both system size and cost.
Designers who remove the snubber may be concerned that the unsnubbed voltage ring will increase the conducted emissions from the unit and require extra filtering.
Figure 1 through 4 show the difference in conducted emissions for the same unit with the snubber fitted and removed. The waveforms show the difference in the leakage inductance spike in both cases.
As you can see, removing the snubber does not cause any measureable difference in conducted emissions at the frequency of the leakage inductance ring (~15MHz).
A combination of the base drive current, the current gain of the transistor and the magnetizing inductance of the flyback transformer dictate the peak power which a BJT flyback can supplype. Together, these parameters must be able to support the primary peak current required to deliver the desired output power at the operating frequency.
POUT for a discontinuous-mode flyback:
where
A BJT used as a switch must be driven into saturation during the on interval to minimize on-state conduction losses. In other words, you must supply the BJT with more base current than is required to generate the collector current, which will be allowed to flow by the primary inductance:
When the base voltage goes below Vth, the time that it takes for excess carriers in the base to recombine, delays the turn-off of the FET. It is desirable to minimize the duty cycle of this turn off delay and for this reason the switching frequency range on BJTs is limited, typically to about 60kHz or so.
Ideally, the base current supplied is such that the device is just passing from the saturation region to the active region at the end of the on time, reducing the number of excess carriers at turn off and reducing the turn-off delay.
The more complicated drive required by BJTs is one reason why MOSFETs have replaced them in many applications. Devices such as TI’s UCC28720 and UCC28722 have addressed this issue by dynamically adjusting the drive current with load. At lower load levels, the reduced base current ensures that there is not a large amount of excess charge in the base region upon turn-off.
These devices also feature a 1W pull resistance on the drive pin to short the base-emitter junction during the off time so that the BJT can sustain the rated collector-to-emitter (VCES) voltage. To sustain the VCES rating, it is important to short the base node to ground through a low-impedance connection during the off time, and that the collector current has stopped flowing before the collector voltage rises above Vceo to avoid second breakdown.
The UCC28720 and UCC28722 simplify the drive for system designers and make power BJTs drop-in solutions for low-power flybacks, reducing component count and system cost.
Download the TI Designs Snubberless, Non-Isolated AC/DC Flyback Converter Reference Design with Simplified Transformer
Design using UCC2872x parts.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2023, Texas Instruments Incorporated