Publication date: 07 April 2009
Spurred by new and emerging energy-efficiency regulations, switch-mode power supplies are rapidly replacing traditional linear power supplies – not just in rapidly-changing products, but also in industrial and appliance applications. Historically, the complexity of switch-mode power supplies has been a barrier to penetrating relatively low-volume markets; however, today’s integrated primary-side-control ICs dramatically simplify power supply design.
An example of the new generation of primary side control ICs is the LinkSwitchâ-CV(1) from Power Integrations. LinkSwitch-CV completely eliminates the optocoupler and all secondary-side feedback circuitry. It is suited for auxiliary power supplies in the 3- to 15-watt range where tight voltage regulation is required, such as directly powering a microcontroller without the need for an inefficient linear regulator. Figure 1 illustrates a complete 6 W flyback power supply based on a LinkSwitch-CV implemented with only 24 components. The supply generates a single isolated 5 V, 1.2 A output voltage from 90-264 VAC input voltage range with output regulation ±5% across load, line, and temperature.
Figure 1. Schematic of a 5 V, 1.2 A adapter power supply using the LinkSwitch-CV, LNK625PG
The main primary-side switching element in the circuit is the power MOSFET incorporated in U1. When enabled, the control logic in U1 turns the power MOSFET on at the beginning of each cycle. The MOSFET is turned off when the current ramps up to the current limit. When the controller switches the MOSFET off, the voltage across the transformer T1 windings reverses, the output diode D7 is forward biased, and current begins to flow in the secondary winding, which replenishes the charge in the output capacitors C7 and C8, and supplies current to the load.
U1 regulates the output using ON/OFF control, enabling or disabling switching cycles based on the sampled voltage on the Feedback pin (FB). The output voltage is sensed using a primary referenced winding on transformer T1. The flux on the bias winding of T1 is directly proportional to the flux in the main secondary power winding and thus provides information of the voltage and current present at the load. The resistor divider formed by R3 and R4 feeds the winding voltage into U1. The ratio of R3 and R4 determines the output voltage set point. The control method compensates for variations in the inductor and other component tolerances, as well as input voltage variations. This innovative use of transformer feedback is the key design element in achieving the power consumption and cost goals for the power supply.
The FB pin voltage is sampled 2.5 μs after turn-off of the high-voltage switch. If the sensed voltage is higher than the threshold of 1.86 V, the following switching cycle is disabled. Conversely, if the sensed voltage is lower than the threshold of 1.86 V, the following switching cycle is enabled. In this manner, the controller maintains output regulation by adjusting the ratio of enabled-to-disabled cycles. The ON/OFF control optimizes efficiency of the converter over the entire load range, maintaining a constant efficiency at very light loads. The design achieves a no-load consumption of less than 80 mW at 230 VAC and meets ENERGY STARâ EPS v2.0 active-mode efficiency requirements (77% vs. 70%).
The power supply provides full protection against short-circuit output and other faults. Auto-restart protection built into LinkSwitch-CV generates the output characteristic shown in Figure 2. In the event of an over-current or short-circuit fault detection, U1 enters auto-restart. The power MOSFET is disabled for 2.5 seconds before a restart is initiated. If the fault is still present, the auto-restart alternately enables and disables switching of the power MOSFET with ~8% duty cycle until the fault condition is removed.
Figure 2. Output characteristic envelope
If the sensed FB pin current during the forward period of the conduction cycle (switch “on” time) falls below 120 μA, U1 initiates an open-loop condition response. The auto-restart time is reduced from 200 ms to approximately six clock cycles (90 μs), while keeping the disable period at 2.5 seconds. This effectively reduces the open-look, auto-restart duty cycle to less than 0.01%.
Further security is provided by the fusible flameproof resistor RF1. This limits inrush current at startup and protects against catastrophic breakdown.
The circuit is also shut down in the event of an over-temperature event. When the die temperature rises above 142°C (±5%), the power MOSFET is disabled and remains disabled until the die temperature falls by 60°C, at which point the MOSFET is re-enabled. This hysteretic recovery ensures safe PCB temperatures under all conditions.
The absence of power-wasting secondary CV control circuitry and primary or secondary current sense resistors all contributes to the design simplicity and helps meet international energy efficiency standards (CoC / China (CECP) /EISA / DoE / European Commission). A further simplification is achieved by the T1 secondary winding (Pins 3-5) serving a second function in providing power to U1 via D6 and R5. This is possible because U1 is self biasing and completely powered via the Bypass (BP) pin and decoupling capacitor C4.
The power supply complies easily to EN550022 and CISPR-22 Class B EMI standards with minimal filter cost. This is made possible by a combination of jitter being applied to the switching frequency of the power MOSFET and by employing innovative transformer E Shield™ techniques. The frequency jitter is a built-in function of U1. Differential EMI filtering is provided by RF1, C1, C2, and L1. No Y-capacitor across the primary-to-secondary isolation barrier is required. The primary clamp (D5, R1, R2, and C3) limits the maximum peak drain voltage to less than the 700 V BVDSS rating of the internal MOSFET of U1. Resistor R2 damps out the high-frequency leakage inductance ringing, thereby reducing EMI. Shielding is provided in the transformer T1 by an open-ended winding connected to Terminal 1. For the cost of a few turns of wire, the need for external shielding is completely removed.
Full details of the transformer, printed circuit board, and bill of materials can be found in DI 201(2) and RDR-201(3) available from www.powerint.com.Figure 3. Populated Circuit Board
Table 1. Reliability improvement resulting from reduced component count of the primary-side control design
Until recently, the complexities and subtleties of switch-mode power supply design have tended to limit their usage in the low-power range to high-volume applications such as mobile phone adapters. However, with modern switching ICs employing primary-side control, the design process has been simplified to the extent that switch-mode power supplies are now a viable choice for low-volume applications typically found in the industrial sector. For more information, visit www.powerint.com
1. LNK623-626 LinkSwitch-CV Family, Energy-Efficient, Off-line Switcher with Accurate Primary-side Constant-Voltage (CV) Control. Power Integrations Inc.2. DI-201 Design Idea LinkSwitch-CV 6 W, High Efficiency Adapter Power Supply. Power Integrations Inc.3. RDR-201 Reference Design Report for a 6 W Constant Voltage (CV) Adapter Using LNK625PG. Power Integrations Inc.
