“XDSL modems in home and office applications are usually powered by external AC-DC (AC-DC) adapters. Judging from the usage habits of most people, these adapters have been plugged into the power outlet and continue to consume power from the AC mains power supply. It is estimated that up to 25% of the electrical energy passing through the adapter is consumed during standby (no load). For this reason, the AC-DC adapter must be designed to maintain extremely low energy consumption in standby mode.
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XDSL modems in home and office applications are usually powered by external AC-DC (AC-DC) adapters. Judging from the usage habits of most people, these adapters have been plugged into the power outlet and continue to consume power from the AC mains power supply. It is estimated that up to 25% of the electrical energy passing through the adapter is consumed during standby (no load). For this reason, the AC-DC adapter must be designed to maintain extremely low energy consumption in standby mode.
In addition to keeping the standby energy consumption as low as possible, the working efficiency of the AC-DC adapter must be very high. Since the energy consumption of Electronic devices after all occupies a higher proportion (about 75%), higher work efficiency can help save power. In view of this, regulatory agencies around the world continue to publish and implement energy efficiency requirements for external power supplies (EPS) during work and standby, as shown in Table 1.
Table 1. Some main energy efficiency specifications for external power supplies
Modem AC-DC adapter design specification requirements
For modem original equipment manufacturers (OEM), AC-DC adapters have become a bulk commercial product. Therefore, they mandate compliance with stringent specifications while also requiring low cost. For AC-DC adapters, the key performance indicators include three items, namely: power density (driven by package size requirements), safety, and low case temperature. The specification requirements of ON semiconductor’s modem AC-DC adapter reference design are as follows:
Input: 90-270 Vac, 50/60 Hz
Output: 12 Vdc±5% at 1.3 A continuous current (power is 16 W); 1.6 A surge current up to 10 s
Voltage stabilization: <2% under combined line and load conditions
Output ripple: less than 200 mV p/p
Steady current: <10% under combined line and load conditions
Average energy efficiency: ≥0.09 * Ln (16) + 0.49 = 74% (in line with “Energy Star” external power supply 1.1 requirements)
Standby (no load) energy consumption: ≤0.3 W
Working temperature: 0 to 50°C
Cooling method: convection
Input protection: 1 A fuse is used to provide 8 Ω surge limit
Output protection: over current protection, over Voltage protection and over temperature protection
Comply with EMI standards: FCC Part 15 conducted EMI (Level B, average profile)
The working principle of the circuit
Figure 1 shows the circuit schematic diagram of this AC-DC adapter. It can be seen from the figure that the adapter power supply is designed based on the flyback converter topology, using a simple Zener device, plus an optocoupler feedback circuit for output voltage sensing and stabilization. The AC input is full-wave rectified by 4 diodes from D1 to D4 and filtered by capacitors C3 and C4 to provide a DC “bulk” bus for the flyback converter section. Resistor R1 provides a surge current limiting function when it is turned on, while capacitors C1, C2 and inductors L1, L2 form common mode and differential mode filtering for conductive electromagnetic interference (EMI).
Figure 1. The circuit schematic diagram of ON Semiconductor 16 W Modem AC-DC adapter reference design
The flyback converter is composed of NCP1027 controller (including integrated MOSFET U1), flyback transformer T1, diode D6, capacitors C6 and C7 and other secondary output rectification/filtering parts. The auxiliary winding on T1 and related components such as R15, D7, C10, R9 and C9 provide working bias (VCC) for this control chip, and allow low output power when the power supply is short-circuited, and allow extremely low standby under no-load conditions Energy consumption. Since the voltage generated by the auxiliary winding tracks the main output voltage, this voltage is also used to sense overvoltage conditions when the feedback loop is open.
The overvoltage protection (OVP) trip level can be adjusted by the number of turns of the auxiliary winding and the value of the resistor R9. The main secondary voltage is rectified by Schottky diode D6 and filtered to a relatively normal DC level by main output capacitors C6 and C7. Capacitor C12 provides additional high-frequency noise filtering for the output. The resistor capacitor diode (RCD) buffer composed of R2, R3, C5 and D5 is used to clamp the voltage spike caused by the primary leakage inductance of T1. This snubber network limits the peak voltage and reduces potential EMI emission problems, thereby preventing potential MOSFET drain damage (pin 5).
Figure 2. Non-dissipative resonant snubber circuit that can replace RCD snubber circuit
In addition to the above-mentioned RCD snubber circuit, there is an alternative non-dissipative resonant snubber circuit, as shown in Figure 2. According to the properties of the transformer design and related parasitic parameters, this type of buffer can also increase the circuit efficiency by several percentage points. As the output voltage and/or power level of the power supply decreases—depending on specific needs, this increase in energy efficiency may be critical to meeting Energy Star energy efficiency requirements. This non-dissipative buffer circuit uses a resonant tank circuit composed of Lr and Cr. This resonant tank circuit essentially acts as a reactive charge pump, returning the leakage reactance energy of the transformer to (C4 On the input bus instead of venting it on the resistor. This can be achieved by an additional fast recovery diode and a small 1.5 mH Inductor Lr, but it will increase a little cost.
Return to Figure 1. The output voltage regulation is realized by the combination of components such as Z1, R5, R6, R7 and optocoupler U2. When the output voltage increases to about 12 V, the Zener device Z1 conducts, and when enough current flows into R7 to generate the 0.9 V voltage required to turn on the optocoupler diode, the voltage feedback loop is closed and the output will be regulated . The use of resistor R7 forces the Zener current to become a stable part of the device voltage/current (V/I) curve, minimizing the temperature effect of the output voltage. The output voltage will be equal to the rated Zener voltage plus approximately 0.9 V. However, due to the characteristics of the Zener device and the optocoupler and the small negative temperature system of this circuit, there may be some changes in the (actual voltage), but the output voltage (Vout) set point change must not exceed ±5%. The optional resistor R5 supports fine adjustment of the output voltage only in the upward direction.
If the output current exceeds about 1.8 A, the converter duty cycle will be reduced by the peak current sensing of MOSFET U1, and the output voltage will begin to drop. Since the Vcc bias voltage on C10 will decrease with the output voltage, eventually Vcc pin 1 will not have enough voltage to power the controller, and the power supply will enter the start-stop hiccup (hiccup) mode, which will prevent large output currents Enter the overload condition and protect the power supply and load at the same time.
The network of resistors R10 to R12 provides undervoltage protection for the circuit when the AC input voltage (and the DC buck voltage correspondingly) drops below approximately 75 Vac. The level on pin 3 (the chip is turned off at this pin) can be adjusted by R10. C11 provides filtering for this input. In addition, if necessary, optional resistors R8, R13, and R14 can be used to provide optional over-power compensation.
Transformer design
For low-power applications, the size of the transformer needs to be as small as possible; however, as the size of the transformer becomes smaller, the cross-sectional area of the magnetic core also becomes smaller. This requires more primary turns to maintain acceptable magnetic flux density limits, and may cause too many turns to accumulate on the spool, thereby inhibiting effective insulation between the primary and secondary. Too many primary turns will also increase the primary leakage inductance, which does not mention the DC impedance that is usually present on the winding. The E25/10/6 iron core is used in this reference design, and a satisfactory compromise is made for the above-mentioned parameter problems. The transformer design for universal input is shown in Figure 3.
There is also a design specifically for 230 Vac input conditions (Europe), which can provide higher energy efficiency and increase the continuous power output to 20 W (1.65 A). Regardless of the design, the primary is divided into two layers, and the secondary and Vcc windings are sandwiched in between. This configuration has lower leakage inductance and therefore provides a lower voltage spike when the MOSFET is turned off. This three-winding 12 V secondary is suitable for minimizing AC and DC losses in the windings. The exact pin output will depend on the specific wiring, but the core selection, wiring harness size, inductance value and turns ratio should be suitable for proper operation. This special flyback transformer is designed for 100 kHz discontinuous conduction mode (DCM) operation, so the slope compensation feature provided by pin 2 of NCP1027 is not necessary.
Figure 3. Transformer design for universal input conditions (90-270 Vac)
Test Results
1) Work efficiency.
Table 2 shows the energy efficiency test results at 25%, 50%, 75% and 100% load under 120 and 230 Vac input conditions. The left table shows the energy efficiency data of the reference design using the RCD snubber circuit, and the right table shows the relatively higher energy efficiency data of the reference design using the resonant snubber circuit. It is worth mentioning that the average energy efficiency in these two cases easily meets the requirements of the CEC and Energy Star EPS specifications (version 1.1) for their power level ranges. Under the 230 Vac input condition, the energy efficiency will be slightly reduced at light load, mainly because the switching loss of the MOSFET is higher at this input level.
Table 2, The average energy efficiency of ON Semiconductor 16 W Modem AC-DC adapter reference design
2) Standby (no load) energy consumption
The no-load energy consumption of the reference design using the traditional RCD snubber circuit is:
290 mW @ 120 Vac
210 mW @ 240 Vac
No-load energy of the reference design with non-dissipative resonant buffer circuit:
240 mW @ 120 Vac
200 mW @ 240 Vac
These no-load energy consumption data not only meet the requirements of CEC and “Energy Star” version 1.1, but also meet the latest “Energy Star” version 2.0 requirements.
Summarize
This article introduces a fully constructed and tested GreenPointTM solution used by ON Semiconductor for xDSL modem AC-DC adapters. This power reference design is intended for low-altitude offline applications that require good output regulation. In addition to xDSL modems, this reference design is also suitable for printers, routers, hubs, and/or similar consumer audio and video applications that require a single output voltage in the range of 10 to 20 W.
This power reference power supply design is based on ON Semiconductor’s NCP1027 monolithic controller integrated with 700 V MOSFETs. It constructs a power supply with an output capacity of 12 V and 1.3 A. It has a surge capability of more than 1.6 A and is in line with “Energy Star”, etc. Standardize the work energy efficiency and standby energy consumption requirements of the organization, as well as other safety regulations. It is worth mentioning that only need to reconfigure the transformer ratio and voltage reference Zener device, this reference design can be modified for output voltage of a few volts up to 28 V (or higher), power is about 20 Application of W.
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