[FAQ] UCC27517A-Q1: Why should I replace discrete gate drivers with low-side driver ICs in automotive PTC modules?

In hybrid electric vehicles/electric vehicles (HEV/EV), the engine is not used to run the heating and cooling system like it is done in the internal combustion engine (ICE) vehicles. To replace this function, two key systems are being used: a BLDC motor to rotate the air conditioner compressor and a positive temperature coefficient (PTC) heater to heat the coolant.

A PTC heater runs off the high-voltage battery and can require several kW of power. Figure 1 shows the block diagram of a typical PTC heater that is driven by a low side MOSFET/IGBT power switch.

Figure 1: Block diagram of an automotive interior heater module

Traditionally, bipolar junction transistor (BJT) totem-poles have been used to drive power switches in low-side configurations. However, gate driver ICs have increasingly replaced these discrete solutions because of the advantages and additional features they provide. Figure 2 shows a typical BJT totem-pole configuration vs. a typical gate driver IC.

Figure 2: BJT totem-pole (left) vs. gate driver IC, UCC27517A-Q1 (right)

One of the biggest shortcomings of a discrete circuit is that it offers no protection, whereas a gate driver IC integrates features that are important to ensure a predictable and robust gate drive. UCC27517A-Q1, which is AEC-Q100 qualified for automotive, comes with built-in undervoltage-lockout (UVLO). This integrated feature clamps the output of the UCC27517A-Q1, preventing switching and drain-to-source voltage across the MOSFET at its output. Once the supply voltage reaches the UVLO rising threshold, the driver will be able to supply current to the power switch.

In contrast, a BJT totem-pole allows a voltage drop across the MOSFET, while the drain current rises significantly. This rise in current leads to excessive power dissipation and can potentially damage the MOSFET.

Figure 3 shows a thermal image of two MOSFETs at 3.3-V start-up. On the left is the MOSFET driven by the UCC27517A-Q1, and on the right is the MOSFET driven by the BJT totem pole. Since the BJT totem-pole doesn’t integrate UVLO, it exposes the MOSFET to overheating due to increased power dissipation.

Figure 3: Thermal image of MOSFET driven by UCC27517A-Q1 (left) and MOSFET driven by BJT totem-pole (right) during 3.3-V start-up

External UVLO circuity can be added to the discrete BJT totem-pole circuit, but it will further increase the number of components leading to an even larger board footprint and higher BOM costs. A gate driver IC, such as the UCC27517A-Q1, requires fewer components and takes up less PCB space compared to a discrete gate drive implementation. Figure 4 highlights the PCB layout of the UCC27517A-Q1 (left) versus the PCB layout of a discrete low-side gate driver (right).

Figure 4: PCB layout of the UCC27517A-Q1 (left) and PCB layout of the discrete low-side gate driver (right)

The UCC27517A-Q1 layout is made up of five components, whereas the BJT totem-pole layout is made up of 10 components. This leads to an area reduction of about 65% when comparing the discrete layout against the gate driver IC layout. A smaller overall layout with fewer components consumes less PCB space, which drives costs down and also increases power density.

For multi-channel solutions, the UCC27624-Q1 is a dual-channel, low-side driver that can be used to drive multiple power switches.

References and additional literature:

• Review the automotive interior heater module interactive system block diagram
• How to design heating and cooling systems for HEV/EVs
• Low-side gate drivers with UVLO versus BJT totem-pole
• Learn more about the UCC27517A-Q1
• Learn more about the UCC27624-Q1
• UCC27712-Q1: Why should I replace 3-phase bridge drivers with half-bridge drivers in automotive AC compressor modules?