Review, teardown, and testing of RS-150-24 Mean Well power supply

General description

A short description. The RS-150-24 is a power supply with a constant output voltage of 24 volts and a current of up to 6.3 amperes. According to the specification, the unit has two AC input voltage operating ranges—from 88 to 132 and 176 to 264 volts. Range selection is non-automatic with a mechanical switch. The supply measures close to 7.5 × 4.0 × 1.5 inches (192 × 98 × 38 millimeters) and is made on a printed circuit board fixed to the base of the metal case, designed to operate with passive cooling. The top lid covering the case is perforated.
The power supply has an LED indication for the output voltage, allowing one to adjust it within -5 to +10%. This unit does not have either PFC or thermal protection.

Design description. The input and output circuits of the power supply are connected to a common screw block (1). From left to right, there are three terminals for the input line, neutral and ground wires, and two parallel blocks of two terminals for the outputs: ground and +24V.
The input voltage from the screw terminals through the fuse (2) is supplied to the RF interference filter (3) and then goes to the diode bridge (5). A varistor (4) is installed at the filter input to suppress hazardous pulses. The rectified voltage from the bridge (5) through the range selector (6) and through two NTC inrush current limiters (7) is supplied to the input electrolytic capacitors (8).
Rectified and filtered voltage from capacitors (8)
goes to the forward converter, which consists of a NE1101 controller (9, on the back side of the board), a 2SK3878 power N-MOSFET transistor (10), and a transformer (11). The voltage from the output winding of the transformer (11) is rectified by the fast-recovery diode 20F20SAB3 (12) and filtered using an output LC filter (13) (14).

The base resistance of each NTC is about 4.5 Ohms.

Output filter capacitance: 2 pieces of 470 uF, 35 volts, designed for operating temperatures up to 220℉ (105℃) (14).

The output voltage is stabilized by shunt regulator AS431ANTR-E1, transmitting the control signal to the high-voltage side of the circuit through the 817C transistor optocoupler (15). A second optocoupler of the same type forms a bypass channel for overvoltage protection (OVP).

The rectifier bridge (5), transistor (10), and diode (12) are installed with individual heat sinks, which (10, 12) are pushed against the housing with screws. Between the aluminum case and the board (from the solder side) is an extra insulation layer, a thin fiberglass sheet. All bulky components are additionally fixed using a compound.

Build quality is good.
The board has empty spaces for installing an additional parallel diode (12) and three output electrolytic capacitors. The board is obviously unified for all the models in the series, and these elements are used in lower-voltage models.

The output voltage LED indicator (16) and the output voltage adjustment resistor (17) are located near the terminal block so that they can be accessed without removing the top cover.

Test conditions

Most tests are performed using Metering Setup #1 (see appendices) at 80℉ (27℃), 70% humidity, and 29.8 inHg pressure.
Unless mentioned otherwise, the measurements were performed without preheating the power supply with a short-term load.
The following values were used to determine the load level:

Output voltage under a constant load

The high stability of the output voltage should be noted.

Power-on parameters

Powering on at 100% load

Before testing, the power supply is turned off for at least 5 minutes with a 100% load connected.
The oscillogram of switching to a 100% load is shown below (channel 1 is the output voltage, and channel 2 is the current consumption from the grid):

On the oscillogram, three phases of the starting process can be distinguished:
1. Two pulses of the input current charging the input capacitors when connected to the grid have an amplitude of about 14.5 A and a duration of about one main voltage period.
2. Waiting for the power supply control circuit to start for about 220 ms.
3. (Output Voltage Rise Time) Output voltage rise takes 5 ms.
(Turn On Delay Time) The entire process of entering the operating mode from the moment the device powers on is 228 ms.

(Output Voltage Overshoot) The switching process is aperiodic; there is no overshoot.

Powering on at 0% load

The power supply is turned off at least 5 minutes before the test, with a 100% load connected. Then, the load is disconnected, and the power supply is switched on.
The oscillogram of switching to a 0% load is shown below:

The picture shows three distinguishable phases of the power-on process:
1. Charging the input capacitors when connected to the grid has an amplitude of about 14.5 A.
2. Waiting for the power supply control circuit to start for about 228 ms.
3. (Output Voltage Rise Time) Starting the converter, increasing the output voltage, and entering the operating mode takes 5 ms.
(Turn On Delay Time) The entire process of entering the operating mode from the moment the device powers on is 233 ms.

(Output Voltage Overshoot) The switching process is aperiodic; there is no overshoot.

Power-off parameters

The power supply was turned off at 100% load, and the input voltage was nominal at the moment of powering off. The oscillogram of the shutdown process is shown below:

The oscillogram shows two phases of the shutdown process:
1. (Shutdown Hold-Up Time) The power supply continues to operate because the input capacitors hold charge until the voltage across them drops to a certain critical level, at which point maintaining the output voltage at the nominal level becomes impossible. The phase takes 38 ms.
2. (Output Voltage Fall Time) Reduction of the output voltage, stopping voltage conversion, and accelerating the voltage drop takes 33 ms.

(Output Voltage Undershoot) The shutdown process is aperiodic; there is no undershoot.

The amplitude of the current at 100% load before shutting down is 5.7 A.

Output voltage ripple

100% load

At 100% load, the low-frequency ripple is approximately 3 mV.

At 100% load, the ripple at the converter frequency is approximately 40 mVp-p, and the noise is 100 mVp-p.

75% load

At 75% load, the low-frequency ripple is approximately 4 mV.

At 75% load, the ripple at the converter frequency is approximately 40 mVp-p, and the noise is 100 mVp-p.

50% load

At a 50% load, the low-frequency ripple is approximately 3 mV.

At 50% load, the ripple at the converter frequency is approximately 25 mVp-p, and the noise is 100 mVp-p.

10% load

At 10% load, the low-frequency ripple is approximately 3 mV.

At a 10% load, the ripple at the converter frequency is approximately 40 mVp-p, and the noise is 100 mVp-p.

0% load

No-load current consumption measured with a multimeter: 68 mA.
(Power Consumption) The current consumption in this mode is predominantly reactive, so it isn't easy to reliably measure it with a basic set of instruments. The power supply's input filter contains two capacitors with a combined capacitance of approximately 1 uF.

At 0% load, the low-frequency ripple is indistinguishable from background noise of approximately 2 mVp-p.

At 0% load, the ripple at the converter frequency is masked by the background noise of approximately 50 mVp-p.

Dynamic characteristics

A mode with periodic switching between 50% and 100% load was used to evaluate the dynamic characteristics. The oscillogram of the process is shown below:

It is evident that the supply’s response to abrupt load changes is aperiodic; the magnitude of the response to load changes is about 100 mV p-p.

Overload protection

The claimed protection type is "hiccup mode, which recovers automatically after the fault condition is removed." This was confirmed during testing. When a short circuit occurs, the power supply periodically tries to turn back on and, if the overload is still present, turns off again until the next attempt.

The output current for the overload protection to kick in is 8.8 A.

Input circuit safety assessment

(Input discharge) Safety assessment is based on the discharge time constant of the input circuits when disconnected from the grid; the value is 0.234 s. This means that when operating on a 120 V input voltage, the time required to discharge the input circuits to safe values (<42 V) will be 0.652 s:

Important: The result is valid for this particular power supply unit; it was obtained for testing purposes and should not be taken as a safety guarantee.

The leakage current at the ground pin is 29 µA.

Thermal conditions

When operating with no load connected, no component overheating had been noticed.
Thermograms were captured at three power levels: 80, 90, and 100%, fully assembled and with the lid removed. Thermal images show that the most loaded element of the block are four ballast resistors that shunt the source output, which are located near the inductance of the output LC filter (13) and whose heating noticeably stands out against the background of other components. At 80% load, they heat up to 219℉ (104℃, 139℉ above ambient temperature). At 90%, it's 233℉ (112℃, 153℉ above ambient), and at 100%, it reaches 239℉ (115℃, 159℉ above ambient).
It is worth noting here that overheating increases faster than output power.

80% load

90% load

100% load

Conclusions

RS-150-24 generally has little noise and ripple, the output voltage is maintained accurately, and the build quality is solid.
The power supply's dynamic characteristics are fine; the unit reacts to a pulsing load with no overshoot.

According to the specification, it is designed for “cooling by free air convection” and “high operating temperatures up to 70°C.” However, our test unit at 100% load heated up its load resistors up to 320℉ (160℃), which seems dangerous. For long-term operation, the load should be limited to 70–80% of the nominal one, especially during the hot season when ambient temperatures reach 95℉ (35℃) or more.

When assessing the safety of the operation of such a power supply, it is necessary to consider the possibility that the load exceeds the rated value due to malfunction but remains below the protection trigger level. In this case, the output power for the tested unit will be 135% of the nominal value, leading to even greater overheating, resulting in power supply failure and a fire hazard.

Important: The results are valid for this particular power supply unit; they were obtained for testing purposes and should not be used to evaluate all the units of the same type.