Evaluation of EMC Emissions and Ground Techniques on 1- and 2-layer PCBs with Power Converters

December 1, 2021

Figure 1: Top-level schematic – functional blocks

Part 7: AC/DC Converter Design with EMC Considerations

This is the seventh column in a series devoted to the design, test, and EMC emissions evaluation of 1- and 2-layer PCBs that contain AC/DC and/or DC/DC converters, and employ different ground techniques [1-6].

Figure 1 shows the functional blocks of the PCB assembly [1].

This column is devoted to the design of the AC/DC Off-Line Flyback Converter. We present a schematic and PCB layout along with the EMC considerations and supporting design documentation.

1. AC/DC Converter Schematics and Design Requirements

Figure 2 shows the block-diagram AC/DC converter schematic.

Figure 2: AC-DC converter block-diagram schematic

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The detailed schematic of the converter is shown in Figure 3.

Figure 3: AC-DC converter detailed schematic

The AC input filter consists of a common-mode choke (L3), line-to-ground Y-caps (C13, C1, and C14, C17 and C18), and line-to-line X-caps (C15 and C16).

The filtered signal is fed to the full-wave bridge rectifier which produces a pulsating positive AC waveform. This waveform is smoothed by the LC filter consisting of L1, C2, C3, and R2 (R2 damps the antiresonant behavior of the filter). The output of the filter is a DC signal of the value 115 × √2 ≈ 162 V. This signal is fed to the primary winding of the flyback transformer. R1 and C1 on the primary side constitute a protective circuitry for the primary winding when the MOSFET switches off. R14 and D5 constitute a protective circuitry for the MOSFET. R17 and C27 constitute a snubber across the MOSFET.

PWM Transistor Q1, controlled by U3 [7], opens and closes the connection to the transformer primary. This switching of the line voltage generates a current on the secondary side and on the auxiliary winding. The winding ratios of the transformer were chosen to generate 24V on the secondary side and 19V on the auxiliary winding. Auxiliary voltage powers U3. Inductor FB2 and C7 on the auxiliary side provide filtering.

Series combination of R3 and R5 constitutes a start-up resistor that charges a reservoir capacitor C7. VCC is internally regulated down from VIN. It is decoupled to ground with a capacitor C9. VIN is provided by the auxiliary winding of the transformer. Initially, both VIN and VCC are 0V. After the line voltage is applied, the current flowing through the start-up resistor charges capacitor C7. Subsequently, the internal regulator charges capacitor C9 (at this point the switching transistor is off and thus the auxiliary winding voltage is zero). Charging of C9 stops when VCC reaches approximately 9.5v, while the voltage across C7 continues rising until it reaches the wakeup level of 24V. Once VIN exceeds the Undervoltage Lockout (UVLO) wakeup level of 24V, NDVR begins switching MOSFET providing energy to the secondary and auxiliary windings. If the voltage on the auxiliary winding builds up to higher than 10V (UVLO lower threshold), then the start-up has been accomplished and sustained operation commences. To sustain the operation of the IC, VIN voltage must be in the range of 11–28V. In our design we chose 19V. This voltage is provided by the auxiliary side of the transformer and is determined by the winding ratio between the primary and auxiliary sides. Decoupling capacitor C20 is connected to VIN.

The primary and secondary side grounds are connected through the Y-rated stitching capacitors C21 and C23.

The voltage on the secondary side is sampled for feedback to U3 through the optocoupler U1. The output voltage set point is determined by the shunt regulator U2 and resistor divider, R6 and R8. The output voltage is given by the following equation

(1)

where V_ref = 1.24 V. R7 and C8 constitute a snubber across the regulator.

Output voltage filtering is provided by C5, C19, and C4. Lastly, there is a bank of 6 LEDs and a 150 Ω resistor to load the output, and there is a 2 pin screw terminal so any other load can be added. This amounts to 3 Watts of power dissipated in the load.

The top layer of the PCB used to create the AC/DC converter is shown in Figure 4, while the bottom layer is shown in Figure 5.

Figure 6 shows the AC/DC PCB converter populated with the components.

Figure 6: AC-DC converter PCB with components

2. EMC Considerations

Similar to the DC-DC converter discussed in Part 2 of this series [2], provisions were added to this design that allow us to add/change components with the goal of improving the EMC performance of the design. These EMC considerations are shown in dashed boxes, labeled A through E in Figure 2.

These considerations are addressed below.

EMC-A: Provisions for an AC input filter were added to the input of this device. This filter consists of a common mode choke (L3), 0.1uF X-capacitors (C15 and C16), and 0.022uF Y-Capacitors (C13, C14, C17, and C18). The goal of this filter is to filter out the noise generated by the switching circuit and propagating out onto the power cord of the device.

EMC-B: A PI-filter was added right after the full bridge rectifier, this filter serves the purpose of smoothing out the rectified AC voltage, but this also provides some additional input filtering. This filter is comprised of a 470uF inductor (L1), a 1.2kΩ resistor (R2), and two 10uF capacitors (C2 and C3).

EMC-C: A pair of gate drive resistors (R9 and R14) and a diode (D5) were added in series with the connection between pin 3 of the switching controller (U3) and the N-Channel MOSFET (Q1). These components allow us to separately control the rise and fall times of this gate drive signal to allow us to slow down the rise and fall times of the signal, thus reducing the high-frequency content of the signal. The diode allows us to control the rise and fall times separately to ensure that the rise and fall times are equal.

EMC-D: Snubber circuits were added to both the N-Channel MOSFET (Q1) and the catch diode (D2). The snubber circuit on the N-Channel MOSFET, comprised of C27 and R17, is used to control the ringing from the MOSFET that results from the step response to the RLC network. The snubber circuit on the catch diode, comprised of C28 and R18, is added across the catch diode to reduce ringing across the diode junction.

EMC-E: Place holders for stitching capacitors (between primary and secondary ground) are added (C21 through C26). These allow us to evaluate the number, placement of capacitors as well as different types of capacitors.

For the next column, we will perform baseline radiated and conducted EMC emissions measurements with a minimal number of EMC components populated. Based on the measurement results we will investigate EMC countermeasures needed to pass; this will be discussed in the subsequent column.