HPH Series

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MDC_HPH_A12 Page 6 of 12

Floating Outputs

Since these are isolated DC/DC converters, their outputs are “floating” with

respect to their input. The essential feature of such isolation is ideal ZERO

CURRENT FLOW between input and output. Real-world converters however do

exhibit tiny leakage currents between input and output (see Specifications).

These leakages consist of both an AC stray capacitance coupling component

and a DC leakage resistance. When using the isolation feature, do not allow

the isolation voltage to exceed specifications. Otherwise the converter may

be damaged. Designers will normally use the negative output (-Output) as

the ground return of the load circuit. You can however use the positive output

(+Output) as the ground return to effectively reverse the output polarity.

Minimum Output Loading Requirements

These converters employ a synchronous rectifier design topology. All models

regulate within specification and are stable under no load to full load condi-

tions. Operation under no load might however slightly increase output ripple

and noise.

Thermal Shutdown

To prevent many over temperature problems and damage, these converters

include thermal shutdown circuitry. If environmental conditions cause the

temperature of the DC/DC’s to rise above the Operating Temperature Range

up to the shutdown temperature, an on-board electronic temperature sensor

will power down the unit. When the temperature decreases below the turn-on

threshold, the converter will automatically restart. There is a small amount of

hysteresis to prevent rapid on/off cycling. The temperature sensor is typically

located adjacent to the switching controller, approximately in the center of the

unit. See the Performance and Functional Specifications.

CAUTION: If you operate too close to the thermal limits, the converter may shut

down suddenly without warning. Be sure to thoroughly test your application to

avoid unplanned thermal shutdown.

Temperature Derating Curves

The graphs in the next section illustrate typical operation under a variety of

conditions. The Derating curves show the maximum continuous ambient air

temperature and decreasing maximum output current which is acceptable

under increasing forced airflow measured in Linear Feet per Minute (“LFM”).

Note that these are AVERAGE measurements. The converter will accept brief

increases in temperature and/or current or reduced airflow as long as the aver-

age is not exceeded.

Note that the temperatures are of the ambient airflow, not the converter itself

which is obviously running at higher temperature than the outside air. Also note

that very low flow rates (below about 25 LFM) are similar to “natural convec-

tion”, that is, not using fan-forced airflow.

MPS makes Characterization measurements in a closed cycle wind tunnel with

calibrated airflow. We use both thermocouples and an infrared camera system

to observe thermal performance. As a practical matter, it is quite difficult to

insert an anemometer to precisely measure airflow in most applications.

Sometimes it is possible to estimate the effective airflow if you thoroughly un-

derstand the enclosure geometry, entry/exit orifice areas and the fan flowrate

specifications. If in doubt, contact MPS to discuss placement and measurement

techniques of suggested temperature sensors.

CAUTION: If you routinely or accidentally exceed these Derating guidelines, the

converter may have an unplanned Over Temperature shut down. Also, these

graphs are all collected at slightly above Sea Level altitude. Be sure to reduce

the derating for higher density altitude.

Output Overvoltage Protection

This converter monitors its output voltage for an over-voltage condition using

an on-board electronic comparator. The signal is optically coupled to the pri-

mary side PWM controller. If the output exceeds OVP limits, the sensing circuit

will power down the unit, and the output voltage will decrease. After a time-out

period, the PWM will automatically attempt to restart, causing the output volt-

age to ramp up to its rated value. It is not necessary to power down and reset

the converter for this automatic OVP-recovery restart.

If the fault condition persists and the output voltage climbs to excessive levels,

the OVP circuitry will initiate another shutdown cycle. This on/off cycling is

referred to as “hiccup” mode. It safely tests full current rated output voltage

without damaging the converter.

Output Fusing

The converter is extensively protected against current, voltage and temperature

extremes. However your output application circuit may need additional protec-

tion. In the extremely unlikely event of output circuit failure, excessive voltage

could be applied to your circuit. Consider using an appropriate fuse in series

with the output.

Output Current Limiting

As soon as the output current increases to approximately 125% to 150% of

its maximum rated value, the DC/DC converter will enter a current-limiting

mode. The output voltage will decrease proportionally with increases in output

current, thereby maintaining a somewhat constant power output. This is com-

monly referred to as power limiting.

Current limiting inception is defined as the point at which full power falls below

the rated tolerance. See the Performance/Functional Specifications. Note

particularly that the output current may briefly rise above its rated value. This

enhances reliability and continued operation of your application. If the output

current is too high, the converter will enter the short circuit condition.

Output Short Circuit Condition

When a converter is in current-limit mode, the output voltage will drop as

the output current demand increases. If the output voltage drops too low, the

magnetically coupled voltage used to develop primary side voltages will also

drop, thereby shutting down the PWM controller. Following a time-out period,

Figure 3 – Measuring Output Ripple and Noise (PARD)

C1

C1 = 0.1μF CERAMIC

C2 = 10μF TANTALUM

LOAD 2-3 INCHES (51-76mm) FROM MODULE

C2

6

5

COPPER STRIP

COPPER STRIP

SCOPE

+OUTPUT

+SENSE

9

8

-SENSE

-OUTPUT