2.5kV H-bridge power design

[julianodelan]

I'm currently designing a dual active bridge with TO 247-4 SiC MOSFETs, connected to a 2.5kV DC-link. I think that a PCB design for the power connections is not the best idea, also because i am not entirely sure if the distance between drain and source pins is enough for that voltage level. Even i was considering to remove the drain pin and just use the backside of the MOSFET. Should i consider mounting it with cooper bars? or from the thermal and the isolation perspective, a thicker PCB design should be enough?
Thanks in advance!

 

[KlausST]

Hi,

Why making a secret about the SiC MOSFET type? .
Why not posting a link to the datasheet?

Now all we can do is guessing.

I guess .. the datasheet give some design informations.
Additionally I guess the manufacturer provides additional documents like "design notes"

Besides the MOSFET package ... the need distances (creepage, clearance) depends on the application :
* safety level
* use case
* altitude
* dirt/dust
* and so on

Generally speaking: if there is a device rated for 2500V (or more? we don´t know) in a THM package .. then I expect the manufacturer will have cared about the voltage.
In other words I expect it to be able to work soldered on a PCB.

Klaus

 

[julianodelan]

My apologies, indeed, the device is rated for 3.3 kV, but I couldn't find any application notes from the manufacturer. Additionally, checking some creepage and clearance tables and calculators, the 5mm pin distance of the TO 247 package may not be enough.

 

[Raina Yin]

I'm currently designing a dual active bridge with TO 247-4 SiC MOSFETs, connected to a 2.5kV DC-link. I think that a PCB design for the power connections is not the best idea, also because i am not entirely sure if the distance between drain and source pins is enough for that voltage level. Even i was considering to remove the drain pin and just use the backside of the MOSFET. Should i consider mounting it with cooper bars? or from the thermal and the isolation perspective, a thicker PCB design should be enough?
Thanks in advance!

For your dual active bridge design with TO 247-4 SiC MOSFETs connected to a 2.5kV DC-link, there are several considerations regarding PCB design and the use of copper bars.

PCB Design Concerns

1. **Voltage Isolation**: The distance between drain and source pins on a PCB is crucial, especially at high voltages like 2.5kV. Standard PCB spacing might not provide sufficient isolation, risking arcing and breakdown.

2. **Thermal Management**: High-power designs generate significant heat. PCBs, especially thicker ones with heavy copper, can help dissipate some heat but might not be sufficient for high-power applications. The thermal conductivity of PCBs is generally lower than that of metals like copper.

Copper Bars Advantages

1. **Better Conductivity**: Copper bars provide significantly better electrical and thermal conductivity compared to PCB traces, reducing resistive losses and improving efficiency.

2. **Thermal Management**: Copper bars can handle higher thermal loads, providing better heat dissipation and helping to keep the MOSFETs cooler. This is especially important for maintaining the reliability and performance of SiC MOSFETs.

3. **Mechanical Stability**: Copper bars can offer more robust mechanical support for high-power connections, reducing the risk of mechanical failures.

Recommendations

1. **Use of Copper Bars**: Given your voltage and power levels, using copper bars for the main power connections is advisable. This will ensure adequate isolation, better thermal management, and improved reliability.

2. **Mounting Considerations**:
- **Drain Connection**: You can remove the drain pin and use the backside of the MOSFET for the drain connection. This method can enhance thermal performance by allowing direct mounting to a heatsink.
- **Source and Gate Connections**: Ensure that the source and gate connections are made with appropriate isolation and spacing to prevent any high-voltage arcing.

3. **PCB for Control Signals**: You can still use a PCB for lower-power control signals and gate drivers, ensuring that these are properly isolated from the high-voltage sections.

4. **Thermal Interface Materials**: Use high-performance thermal interface materials between the MOSFETs and the heatsink to improve thermal conductivity and reduce thermal resistance.

5. **Design Validation**: Perform thorough simulations and possibly build a prototype to validate the thermal and electrical performance of your design before full-scale implementation.

By incorporating these considerations, you can achieve a robust and reliable design for your dual active bridge converter.

 

[KlausST]

Hi,

typical design strategies are:
* using inner layers
* using small, non circle pads
* bending the leads
* milling slots to increase creepage distance
* potting

Klaus

 

[FvM]

Package creepage distance is marginal for 2.5kV. You'll find modified TO247-4 packages from other manufacturers with increased creepage.

PCB design surely needs milled slots. It should also consider effects like electro migration and CAF (conductive anodic filament) growth that limit the inner voltage strength of PCB. Ask your PCB manufacturer for HV capable materials.

PCB creepage distance is particularly critical if your circuit implements safety related isolation.