Galvanically isolated interfaces are a common requirement within industrial devices for safety reasons. In this type of application, a digital isolator is used to galvanically isolate an MCU from a communication transceiver or an ADC.
Digital isolators work at two power domains, using an isolated DC power supply in one domain. The low DC voltage for the isolated power domain could be achieved with a small and simple push-pull converter. The push-pull converter is a transformer-isolated topology using two transistors switching in complementary mode.
This project will present a low-cost and low-power DC/DC push-pull converter based on the Dialog GreenPAK™ SLG46108 device. The following sections will show how to:
● Generate a complementary PWM with dead time using the programmable delay block.
● Generate a start-up sequence using a pipe-delay block.
● Obtain multiple clock frequencies via an internal oscillator.
Below we described steps needed to understand how the DC/DC converter has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.
Operation Principle of Push-Pull Topology
A push-pull converter schematic topology is shown in Figure 1. This converter uses a transformer with center tap in the primary and secondary windings. Two transistors (Q1 and Q2) work to switch the DC input voltage VIN in alternate half-cycles.
The PUSH command is HI during the one-half cycle. The PULL command is high during the other half cycle. When the PULL command is high, Q1 transistor is turned on and the current flows from VIN to Q1 in the primary, and from D2 to the output capacitor in the secondary.
The current flows in the same direction in the output capacitor during both current cycles, keeping a positive output voltage VOUT. The converter output voltage could be given by equation (1)
VOUT = 2 ∗ D ∗ VP ∗ n − VDIODE (1)
Where VOUT is the output voltage, D is the duty cycle, VP is the voltage in the transformer primary winding, n is the transformer turn-ratio and VDIODE is the voltage drop in outputs at diodes (D1 and D2). Vp is given by equation (2):
VP = VIN − Vds (2)
where Vds is the voltage drop from the internal resistance of transistors Q1 and Q2. An issue with this topology is the variation on output voltage with load current change. To ensure a stable output voltage a linear regulator should be added to the output, and the converter output voltage must be higher than the minimum specified for the regulator. The PUSH and PULL command signals are shown in Figure 2. Command signals are complementary and should have the same duty cycle to avoid transformer core saturation.
An important aspect for push pull converters is the need for a short time interval where both commands are low, as can be seen in Figure 2. This time interval is required to avoid the short circuit of both primary ends of the transformer.
The transistors Q1 and Q2 require a small amount of time to effectively turn-on and turn-off. The MOSFET turn-on and turn-off involves a process of charging and discharging a MOSFET gate. A common approach is to model MOSFET gate charge influence as capacitors between MOSFET source and drain. This is shown in Figure 3.
In datasheets, the turn-on and turn-off transitions are presented as shown in Figure 4. However, the switching times are highly affected by circuit conditions, such as gate drive resistance, drain-source...Read more »