12-VAC to 28 VAC, 7.5-W to 25-W Ideal Diode Full-bridge Rectifier Reference Design

Description

This reference design implements the LM74610-Q1 Zero-I_Q Reverse-Polarity Protection Smart Diode Controller to control the high-side rectifier MOSFETs for both line and neutral inputs of a 12-V_AC to 28-V_AC power supply. The design was tested at loads from 7.5-W to 25-W. The use of low R_DS(on) MOSFETs with ideal diode drivers eliminates the high losses found in standard diode rectifiers. Applications include video doorbells and smart thermostats that run off the 12-V or 24-V_AC transformer of a home or building.

1 Test Prerequisites

1.1 Voltage and Current Requirements

Table 1-1 Voltage and Current Requirements

Parameter	Specifications
Input Voltage	12-V_AC to 28 V_AC
Output Power	7.5-W to 25-W

2 Design Description

The LM74610-Q1 device is a controller device that can be used with an N-Channel MOSFET in a reverse polarity protection circuitry. It is designed to drive an external MOSFET to emulate an ideal diode rectifier when connected in series with a power source. A unique advantage of this scheme is that it is not referenced to ground and thus has Zero I_Q. The LM74610-Q1 controller provides a gate drive for an external N-Channel MOSFET and a fast response internal comparator to discharge the MOSFET gate in the event of reverse polarity. This fast pulldown feature limits the amount and duration of reverse current flow if opposite polarity is sensed.

The LM74610-Q1 controller does not have GND pin. During start up, it depends on the forward drop of the body diode to generate the charge pump voltage whenever a load is connected. The charge pump has a kind of hysteretic control, where it monitors charge pump (VCAP) voltage for high (VCAP-H) and low thresholds (VCAP-L). There will be a phase when gate drive is disabled when VCAP voltage reaches VCAP-L threshold and re-enabled when VCAP voltage is again charged to VCAP-H (see Figure 2-1 and Figure 2-2). For additional information, see the Feature Description section of the LM74610-Q1 Zero IQ Reverse Polarity Protection Smart Diode Controller Data Sheet.

Figure 2-1 LM74610-Q1 Functional Block Diagram

Figure 2-2 V_CAP Charging and Discarding by the Charge Pump

Figure 2-3 highlights the Vgate illustrating VCAP discharging cycles.

Figure 2-3 Vgate Illustrating VCAP Discharging Cycles