This article is intended to introduce and explain the workings of linear and switchmode power supply regulators used on HDD PCBs.
A regulator or DC-DC converter is a component or circuit which takes a voltage input which can vary over a wide range, and converts it to a steady DC voltage which remains stable over wide variations in load current. The input voltage may be stepped down, or boosted, or inverted.
Linear regulators reduce ("step down") an input voltage to a stable lower DC voltage by dissipating the voltage difference internally as ohmic losses. That is, any excess power is dissipated within the regulator as heat.
For example, if Vin is 5V, Vout is 3.3V, and the load current is 1A, then the load dissipates 3.3 watts. The regulator dissipates 1.7W, ie (5V - 3.3V) x 1A. This means that the efficiency of the regulator is only 66%, with 34% of the power being wasted as heat.
A switchmode regulator steps down the input voltage by means of Pulse Width Modulation (PWM). Simply speaking, the input voltage is switched rapidly on and off (pulsed), and the output is then averaged to provide smooth DC.
For example, let's say that we want to reduce a +5V input to +2.5V. We could switch ("chop") the input on and off with a duty cycle of 50%. That is, the input would be on half the time and off half the time. Alternatively, if our desired output voltage were to be +1.25V, then the duty cycle would be 25%. Actually it is not as simple as that, but it is useful to think of it in this way.
The ideal switchmode regulator would have an efficiency of 100%. When its on/off switch is on, the voltage across it is zero. When the switch is off, the current through it is zero. This means that the power dissipated in an ideal switch would be zero (Power = Voltage x Current). Furthermore, the regulator's averaging components consist of capacitors and inductors. Since ideal capacitors and inductors are lossless, then they also would dissipate no power.
In practice the efficiency of a switchmode regulator is more like 80% - 90%. The component that handles the switching is often a MOSFET which has a finite ON resistance (RDSon). The Schottky diode also has a substantial ON resistance. Inductors and capacitors also have finite ESR (Equivalent Series Resistance).
In today's HDDs the switching frequency is in the MHz range. This means that very small capacitances can be used to filter the output voltage. That's why the older electrolytic and tantalum capacitors have been replaced by MLCCs (Multi-Layer Ceramic Capacitor). MLCCs also have extremely low ESR ratings and extremely low leakage currents.
Examples of Linear Regulators
One of the simplest linear regulators is a positive 3-terminal device such as the LX8117-33.
It has an input pin (+5V), an output pin (+3.3V), and a Ground pin. These devices are usually internally current limited. This provides a measure of protection against a short circuit on the output. The devices also have on-chip thermal protection which causes them to shut down when they begin to overheat.
Earlier drives had multiple-output linear regulators. The following example (IRU1239SC) has two inputs (+5V and +12V) and three outputs (+8V, +3.3V, +2.6V). The +8V supply was usually required by the preamp, while the 2.6V and 3.3V outputs were the Vcore and Vio supplies for the MCU.
- IRU1239SC_photo.jpg (32.1 KiB) Viewed 7762 times
Later drives incorporated the linear regulator control circuitry within a separate IC, usually the motor controller. The current was carried by an external NPN "pass" transistor, and the voltage feedback was provided by external resistors.
The error amplifier compares the reference voltage (Vref) against the feedback voltage (Vfb). If the output voltage is too low, then the error amp generates a positive error output which drives the transistor harder. Conversely, if the output voltage is too high, then the error amp generates a negative error output which reduces the drive to the transistor.
