Figure 4. Driving the Remote On/Off Control Pin

temperature (See Performance Specifications), a precision temperature sensor

will power down the unit. When the internal temperature decreases below the

threshold of the temperature sensor, the unit will self start.

Output Overvoltage Protection

The output voltage is monitored for an overvoltage condition via magnetic coupling

to the primary side. If the output voltage rises to a fault condition, which could be

damaging to the load circuitry (see Performance Specifications), the sensing cir-

cuitry will power down the PWM controller causing the output voltage to decrease.

Following a time-out period the PWM will restart, causing the output voltage to

ramp to its appropriate value. If the fault condition persists, and the output volt-

ages again climb to excessive levels, the overvoltage circuitry will initiate another

shutdown cycle. This on/off cycling is referred to as “hiccup” mode.

Input Reverse-Polarity Protection

If the input-voltage polarity is accidentally reversed, an internal diode will be-

come forward biased and likely draw excessive current from the power source.

If the source is not current limited or the circuit appropriately fused, it could

cause permanent damage to the converter.

Input Fusing

Certain applications and/or safety agencies may require the installation of

fuses at the inputs of power conversion components. Fuses should also be

used if the possibility of a sustained, non-current-limited, input-voltage polarity

reversal exists. For these converters, slow-blow fuses are recommended with

values no greater than twice the maximum input current.

Trimming Output Voltage

These converters have a trim capability that enables users to adjust the output

voltage over a limited range (refer to the trim equations). Adjustments to the out-

put voltage can be accomplished with a single fixed resistor as shown in Figures

5 and 6. A single fixed resistor can increase or decrease the output voltage

depending on its connection. Resistors should be located close to the converter

and have TCR’s less than 100ppm/°C to minimize sensitivity to changes in

temperature. If the trim function is not used, leave the trim pin open.

Standard units have a “positive trim” where a single resistor connected from

the Trim pin to the +Sense will increase the output voltage. A resistor con-

nected from the Trim Pin to the –Sense will decrease the output voltage.

Trim adjustments greater than the specified trim range can have an adverse

affect on the converter’s performance and are not recommended. Excessive

ment of the output voltage, can cause the overvoltage protection circuitry to

activate (see Performance Specifications for overvoltage limits).

Temperature/power derating is based on maximum output current and voltage

at the converter’s output pins. Use of the trim and sense functions can cause

output voltages to increase, thereby increasing output power beyond the

converter’s specified rating, or cause output voltages to climb into the output

overvoltage region. Therefore:

d rated output power

The Trim pin is a relatively high impedance node that can be susceptible to

noise pickup when connected to long conductors in noisy environments.

Sense Input

sistors. Nevertheless, if sense is not used for remote regulation, the user

must

intended to correct small output accuracy errors caused by the resistive ohmic

drop in output wiring as output current increases. This output drop (the differ-

allowed to exceed 0.5V.

Sense is connected at the load and corrects for resistive errors only. Be careful

where it is connected. Any long, distributed wiring and/or significant inductance

introduced into the Sense control loop can adversely affect overall system stabil-

ity. If in doubt, test the application, and observe the DC/DC’s output transient

response during step loads. There should be no appreciable ringing or oscilla-

tion. You may also adjust the output trim slightly to compensate for voltage loss

in any external filter elements. Do not exceed maximum power ratings.

Current Limiting

When power demands from the output falls within the current limit inception

range for the rated output current, the DC/DC converter will go into a current

limiting mode. In this condition the output voltage will decrease proportionately

with increases in output current, thereby maintaining a somewhat constant power

dissipation. This is commonly referred to as power limiting. Current limit inception

is defined as the point where the full-power output voltage falls below the specified

tolerance. If the load current being drawn from the converter is significant enough,

the unit will go into a short circuit condition. See “Short Circuit Condition.”

Short Circuit Condition

When a converter is in current limit mode the output voltages will drop as the

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

netically coupled voltage used to develop primary side voltages will also drop,

thereby shutting down the PWM controller. Following the specified time-out

period, the PWM will restart, causing the output voltages to begin ramping to their

appropriate values. If the short-circuit condition persists, another shutdown

cycle will be initiated. This on/off cycling is referred to as “hiccup” mode. The

hiccup cycling reduces the average output current, thereby preventing internal

temperatures from rising to excessive levels. This converter is capable of

enduring an indefinite short circuit output condition.

Thermal Shutdown

These converters are equipped with thermal-shutdown circuitry. If the internal

temperature of the DC/DC converter rises above the designed operating

HPQ Series

Isolated High Power Quarter Brick DC/DC Converters

MDC_HPQ Series.A07 Page 7 of 10

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