Dual 15-V, 120-mA Isolated Bias Power Supply Reference Design Using Buck and Push-Pull Converters

1 Description

This isolated power supply provides dual +15-V outputs at 120 mA each. This power supply is designed for biasing a MOSFET-based power stage that generates excitation for a separately excited synchronous motor (SSM). This design uses a combination of the LMR50410-Q1 buck pre-regulator and the SN6505-Q1 open-loop transformer driver with spread-spectrum modulation. The wide input voltage range and flexibility make this design a versatile solution for wide range of applications where isolated power is necessary. This internal reference design is an alternative to the PMP22760 which uses a single-chip solution based on the LM5180-Q1.

Figure 1-1 PCB Top View

Figure 1-2 PCB Bottom View

Figure 1-3 PCB Aerial View

Figure 1-4 PCB Side View

Figure 1-5 Functional Block Diagram

2 Test Prerequisites

2.1 Voltage and Current Requirements

Table 2-1 Voltage and Current Requirements

Parameter	Specifications
Input voltage range	12 V to 15 V (fully tested) 6 V to 36 V (operational)
Output voltage and current	Two isolated outputs, each +15 V, 120 mA
Switching frequency	2.1 MHz (buck), 450 kHz (push-pull)
Topology	Buck pre-regulator + open-loop push-pull
Transformer dielectric strength (WE 750316853)	3125 VAC at 2 seconds, 2500 VAC at 1 minute
Secondary-to-primary capacitance⁽¹⁾	11.5 pF at 100 kHz
Secondary-to-secondary capacitance⁽¹⁾	Approximately 7.5 pF at 100 kHz

(1) Terminals V_OUT1, V_OUT2, V_OUT-BCK are shorted with a jumper for the measurement. The value is taken with an accurate RCL meter. Naturally, the system is disabled.

2.2 Required Equipment

The following test equipment was used during the design development and testing:

Oscilloscopes Rohde & Schwarz RTB2004, RTH1004
Bench multimeters Keysight 34410A, Keithley 2700, Keithley 2100, Keithley 2110
Hand-held multimeters Agilent U1272A, Brymen BM869s
Regulated laboratory power supply E3648A
Electronic load Keithley 2380-120-60