Hi fellow engineers and TI community members,
I’ve been working on optimizing a push-pull converter design (12V input, 5A output) and noticed a trend: many peers are swapping out traditional BJTs for MOSFETs. The efficiency gains seem significant, but the transition isn’t straightforward—there are subtle design challenges I’m grappling with. Let’s dive into the technical details and crowdsource insights from the community.
1. Fundamental Drive Mechanism: Why Does "Current vs. Voltage" Matter in Real-World Designs?
BJTs rely on base current to control conduction (Ic = β·Ib), while MOSFETs use gate voltage to modulate channel resistance. In my tests, a BJT push-pull stage (using TIP31C) showed Vce(sat) = 0.7V, leading to 7W conduction loss at 5A. Swapping to a MOSFET (VBsemi VBTA1220N, Rds(on)=12mΩ at 10V Vgs) cut that to ~1W.
But here’s the question: For low-power applications (<1A), does the BJT’s simpler drive circuit (no need for stable gate voltage) still make it preferable? Or is the efficiency edge of MOSFETs worth the extra gate driver complexity?
2. High-Frequency Performance: When Does Switching Loss Become a Dealbreaker?
My design needs to operate at 500kHz, and BJT switching loss is becoming problematic. With BJTs, turn-off delay (due to charge storage) caused switching loss to exceed 50% of total loss at 300kHz. A MOSFET (VBsemi VBGQA1602, 60V/3mΩ) reduced switching loss by 80% in side-by-side tests, but…
Has anyone experienced unexpected tradeoffs at >1MHz? For example, do MOSFETs’ faster edges introduce EMI issues that BJTs (with slower transitions) avoid? How do you balance switching speed and EMI in high-frequency push-pull topologies?
3. Miller Capacitance: The Hidden Challenge in MOSFET Designs
MOSFETs’ Cgd (Miller capacitance) has been a headache. In my setup, a 100pF Cgd caused gate voltage glitches during switching, leading to false turn-ons. Adding a 10Ω gate resistor helped, but slowed transitions slightly.
What’s your go-to strategy for mitigating Miller effects? Do you prioritize low-Cgd devices (like VBsemi’s VBG series with 50pF Cgd) over driver optimization, or combine both? Any real-world numbers on how much Cgd affects dead-time requirements in push-pull stages?
4. Practical Swap Guidelines: What Specs Get Overlooked?
When replacing a BJT with a MOSFET, I initially underrated the importance of gate voltage. The BJT worked with 5V base drive, but the MOSFET needed 10V Vgs to hit low Rds(on) (VBTA1220N went from 80mΩ at 5V to 20mΩ at 20V).
What other specs do you check first when swapping? Thermal resistance (RθJA)? Gate charge (Qg) for driver sizing? Has anyone encountered reliability issues (e.g., ESD, overvoltage) that BJTs are more tolerant of?
Let’s Discuss!
Whether you’ve stuck with BJTs for specific use cases, fully switched to MOSFETs, or faced unexpected failures in the transition—share your experiences. What’s your rule of thumb for choosing between BJTs and MOSFETs in push-pull circuits? Any TI reference designs or driver ICs (like UCC27517) that simplify this swap?
Looking forward to the insights!
