The purpose of the isolated power supplies in this system is to take the input voltage from the low-voltage side and create a well-regulated output for the components on the high-voltage side while creating an isolated barrier via galvanic isolation between the high-voltage and low-voltage sides of the PTC system. The operating range of the low-voltage input is often around 6V to 16V, while the absolute maximum range can be around 4V to 42V. The designer can ensure that output voltage of the power supply outputs are high enough to satisfy the power supply inputs of the switch drivers. The required output power is dependent on the power needed to drive the power switches as well as power the MCU(s), sensors, comparators and so forth. The output voltage chosen by designers tends to be around 15V to 20V. There are typically two output rails (one high-side and one low-side), but a third output rail can be used to power the lower voltage components (for example, MCUs, sensors, op-amps). A lower voltage output rail can also enable the designer to use a less expensive LDO.

Regardless of the power topology used in the PTC heater module design, an input filter circuit may be needed to feed into the rest of the power supply in order to help it meet EMI requirements. The input filter circuit may have to be edited and customized to fit the characteristics of the specific PTC heater design. Figure 3-4 shows the circuit for the typical undamped input filter, for example. For guidance on designing an input filter, Input Filter Design for Switching Power Supplies is a good resource.

There are plenty of isolated power topologies a designer can choose from, but the main topologies suitable for PTC heater modules are flyback, LLC and push-pull topologies. Other power supplies may not be recommended because the intended power output for these topologies typically exceeds what is needed for PTC heater control modules. The flyback converter, shown in Figure 3-5, may be the most common topology used in automotive applications due to its simplicity and versatility.

Flyback converters can accept a wide range of input voltages, making them suitable for the operating and absolute maximum ranges of the input voltage of the PTC heater module without a pre-regulator. Flyback converters can also supply multiple output voltages, so they can support the one or two rails typically needed in this application. These output voltages can all be regulated with a single control, being the duty cycle. The output voltage can be calculated with Equation 1:

Since the required output rail voltage is not many magnitudes higher than the input voltage, the transformer turns ratio in the flyback can be relatively low. It is recommended that the turns ratio are to be picked such that the maximum duty cycle will not be higher than 50% at V_IN min. Depending on what is the ratio between the two output rails, the designer may have to get a custom transformer since it is not possible to get a perfect turns ratio transformer in the broad market.

Transformer drivers can simplify the design by integrating components of a power supply system into one integrated circuit (IC). One type of transformer driver is a controller, a component responsible for driving the primary side switch of the flyback circuit at the correct duty cycle, the feedback mechanisms needed in the power supply circuit, under-voltage lockout for the input voltage, and possibly overcurrent protection. This topology can be seen in Figure 3-6. An advantage of using a controller is that the designer can control the slew rate used to drive the primary side switch with an external resistor on the gate. This can help control induced EMI. Another benefit is that the designer has the flexibility to choose the switch individually, enabling more current to be driven if necessary. Some systems may also prefer controllers as they grant the designer the flexibility to put the primary side switch on areas of the PCB that are less likely to cause thermal issues. A suitable controller for this application would be the LM34966-Q1 due to its wide input range of 2.97V to 40V (absolute maximum rating of -0.3V to 45V), enabling this device to withstand automotive load dump and cold crank conditions.

Another transformer driver type is the converter, which comes with the same benefits as a controller and the switch integrated to help shrink and simplify the design while reducing cost, as shown in Figure 3-7. However, integrating the switch does prevent the ability to adjust slew rate and controlling EMI induced by it. A converter that can be suitable for flyback applications is the LM25180-Q1 due to its wide input range of 4.5V to 42V (absolute maximum rating of -0.3V to 45V). This device also integrates the auxiliary winding, offering primary side regulation (PSR) for the power supply design, thus simplifying the design of the transformer.

To help improve EMI and decrease the cost of the transformer needed in the PTC heater module design, a designer may opt to use an LLC topology (see Figure 3-8) instead of a traditional flyback design. An LLC can help increase efficiency by using resonant switching, which is switching involving inductors and capacitors to create sinusoidal currents and voltages during the switching periods. This method helps eliminate switch transition losses, thus increasing efficiency. There is also low parasitic capacitance in this kind of topology and great EMI mitigation, thus enabling the designer to use a cheaper transformer than they would in a flyback converter. However, this topology is an open loop, so variations on the input or output won’t have a controlled response. Another functional disadvantage of LLCs is being open loop is that they cannot take a range of input voltages like flyback converters, so they will need a pre-regulator IC like the LM5157-Q1. Converters are also widely used to help simplify LLC designs. A great choice for an LLC converter would be the UCC25800-Q1 as it can help reduce EMI even more by enabling soft-switching.

Push-pull topologies are also suitable for several high voltage automotive applications, including PTC heater control modules. Unlike a flyback converter, which stores energy in an inductor in one phase of the switching cycle and send it to the load in the other phase, push-pull converters use transformer action to transfer power from the primary side to the secondary side. Their topology can be seen in Figure 3-9.

This is done without analog feedback or loop stabilization. It is also an open loop configuration, so it does not need feedback, simplifying the design. A disadvantage that push-pull topologies have is that they lack load regulation. One of key advantages of push-pull topology is the simplified transformer design. Center-tapped transformers are readily available with various turns-ratio avoiding the need for designing a custom transformer. Many a times, you can also find transformers with multiple outputs readily available and if they are not available, designing the transformer is relatively simple involving only two key parameters, the minimum V-t product and turns-ratio. The data sheets of these devices include list of readily available transformers from multiple vendors for the most commonly used input/output voltage rails.

Converters can also be used to help shrink and simplify push-pull designs. The SN6507-Q1 meets the requirements for most high voltage automotive applications as it offers large input voltage (60V absolute maximum) and line regulation.

The SN6507-Q1 can be used for isolating lower voltage logic rails for powering isolators. Using localized isolated power solutions for individual isolators also simplifies the primary isolated power solution’s design while also helping a simpler PCB layout design.

DC/DC modules can be used in PTC heater modules, as well. They integrate the primary side switch and the transformers of the power supply, significantly reducing board space and height as well as simplifying system design (see Figure 3-10). DC/DC modules drastically reduce the number of discrete components needed. These ICs also come with an integrated isolation barrier. DC/DC modules can also help make it easier to implement a distributed power supply architecture, which involves having one power supply per switch driver. This increases system reliability by providing multiple point of loads allowing independent point of failure detection. So, if one power supply fails, the rest of the system can still operate. With this kind of solution, however, the designer may need to implement a pre-regulator if the input voltage range exceeds the DC/DC module’s absolute maximum ratings. The individual IC cost is relatively higher than converter ICs, but the commercial benefits come in the total system cost saved by integrating so many components. These ICs tend to be less efficient, but a significant amount of design time is saved since several discrete component calculations and considerations do not need to be done. A good choice for a DC/DC module in a PTC heater module would be the UCC14141-Q1, especially if the PTC heater module is using an 800V or higher battery due to this DC/DC module’s 5kV_RMS isolation rating. It also provides the designer with a very low-profile isolated power supply solution of 3.55mm in height.