HP1800 in a 48V to Core Power Design

The HP1800 is a DC-DC converter module that steps a nominal 48 V input down to the low-voltage core rail (typically 0.8–1.2 V) used by CPUs, GPUs, and other digital loads. In a direct 48 V to core power design, it serves as the intermediate or point-of-load stage, replacing the older two-stage approach that first dropped 48 V to 12 V and then to the core voltage. This article explains what the HP1800 does in that architecture, how it works, why it is used, and what you need to consider when designing with it.

Formula

The HP1800 is not a simple linear regulator; it is a switching converter, so the basic power relationship is:

P_out = V_out × I_out

and the input power is:

P_in = P_out / η

where:

• V_in – input voltage in volts (typically 48 V DC)
• V_out – output voltage in volts (core voltage, e.g., 0.8–1.2 V)
• I_out – output current in amperes (can be 50–200 A for modern processors)
• η – efficiency of the converter (usually 85–95% for a well-designed module)

The input current is simply P_in divided by V_in. For example, at 48 V input, 1.0 V output, and 100 A load, the output power is 100 W, and at 90% efficiency the input power is about 111 W, so the input current is roughly 2.3 A. That is the key advantage of a 48 V distribution: the input current is low, so I²R losses in the board traces are small.

Worked Example

Using the default values from the calculator block:

• Input voltage = 48 V
• Output voltage = 1.0 V
• Output current = 100 A

Step 1: Output power = 1.0 V × 100 A = 100 W

Step 2: Input power (assuming 90% efficiency) = 100 W / 0.9 = 111.11 W

Step 3: Input current = 111.11 W / 48 V = 2.31 A

So the HP1800 takes about 2.3 A from the 48 V rail and delivers 100 A at 1.0 V. The calculator rounds the input current to 2.31 A, matching this result.

Background: Why 48 V to Core?

Traditional server and telecom power architectures used a 12 V intermediate bus. The power supply unit (PSU) converted AC to 12 V, and then point-of-load (POL) converters stepped that down to the core voltage. However, as processors demanded more current (100 A, 200 A, even 300 A), the 12 V rail had to carry enormous currents, causing significant I²R losses in the motherboard traces and connectors. For example, a 300 A load at 12 V means 25 A per phase if you have 12 phases, but the distribution loss still hurts.

Moving to a 48 V distribution reduces the current by a factor of four for the same power, cutting resistive losses by a factor of sixteen (since P = I²R). The HP1800 is designed to work directly from that 48 V rail, eliminating the separate 12 V intermediate stage. This is sometimes called a “direct 48 V to point-of-load� or “48 V direct conversion� architecture.

How the HP1800 Works

The HP1800 is a fixed-ratio or regulated DC-DC converter module. In a direct 48 V to core design, it typically operates as a step-down (buck) converter, but it may use a resonant or soft-switching topology to achieve high efficiency at high switching frequencies. The module contains the power switches (MOSFETs), inductors, and control circuitry, and it accepts a wide input range (often 36–60 V) to accommodate transient conditions.

The output voltage is regulated to a precise setpoint, often via an I²C or PMBus interface, allowing the system to adjust the core voltage dynamically (DVFS). The HP1800 also provides telemetry – output current, input voltage, temperature – which the system management controller reads to balance load and detect faults.

Because the core voltage is very low (below 1 V), the module must have extremely tight load regulation and fast transient response. A sudden change in CPU activity can cause the current to jump by tens of amperes in microseconds; the HP1800 uses multi-phase interleaving (e.g., 4, 6, or 8 phases) to reduce output ripple and improve transient response.

Choosing an HP1800 for Your Design

When selecting an HP1800 (or a similar module) for a 48 V direct-to-core design, consider these factors:

• Output current rating – match the module’s maximum current to your processor’s peak demand, with some margin (typically 10–20%).
• Input voltage range – ensure the module handles the full range of your 48 V supply, including transients (e.g., 36–60 V).
• Efficiency – look for peak efficiency above 90% at your typical load; check the efficiency curve for light-load and full-load conditions.
• Control interface – if you need dynamic voltage scaling, verify that the module supports PMBus or I²C commands.
• Thermal design – the module dissipates heat; calculate the power loss (P_loss = P_out × (1/η – 1)) and ensure your heatsink and airflow can handle it.
• Form factor – the HP1800 comes in a surface-mount package; check the footprint and height for your board layout.

Common Problems and Misconceptions

One common misconception is that the HP1800 can be used as a simple linear regulator. It is not – it is a switching converter, so you must provide proper input and output capacitance, and you must follow the layout guidelines to avoid noise and instability.

Another issue is confusing the HP1800 with an isolated converter. The HP1800 is non-isolated (the input and output share a common ground), so it is not suitable for applications requiring galvanic isolation.

A frequent problem in 48 V direct designs is the “hold-up� capacitor requirement. When the AC mains drops momentarily, the 48 V rail can sag; the HP1800’s input voltage range must accommodate that sag, or you need bulk capacitance on the 48 V rail to ride through the disturbance.

Also, because the output voltage is so low, the sense lines (Kelvin sensing) are critical. The HP1800 typically provides remote sense pins that must be connected to the load point to compensate for voltage drop in the PCB traces. Neglecting this can cause the actual core voltage to be lower than the setpoint, leading to system instability.

Comparison: HP1800 vs. Traditional 12 V POL

Compared to a classic 12 V intermediate bus architecture, the HP1800 offers several advantages:

• Higher efficiency – eliminating one conversion stage reduces losses; a typical 48 V to 1 V converter can achieve 90–95% efficiency, whereas a 48 V to 12 V plus 12 V to 1 V cascade might be 85–90% total.
• Lower distribution losses – the 48 V rail carries much less current, so thinner traces and smaller connectors can be used.
• Smaller board area – fewer stages mean fewer components, though the modules themselves may be larger due to the high current.
• Better transient response – a direct conversion can react faster to load steps because there is no intermediate bus capacitor to charge.

However, the 12 V approach has a larger ecosystem and may be cheaper for lower-power applications. The HP1800 is best for high-current, high-performance systems where efficiency and density are critical.

Cost and Availability

The HP1800 is a specialized component, so its cost is higher than a generic 12 V POL converter. Prices vary widely depending on quantity, distributor, and specifications. For a single unit, you might pay anywhere from $20 to $80, but for volume orders the price drops significantly. The total system cost also includes the PCB layout, additional capacitors, and thermal management, so the HP1800 may be cost-effective only when the power savings justify the premium.

Availability is generally good through major distributors like Mouser or Digi-Key, but lead times can be long during supply shortages. Always check the manufacturer’s datasheet for the exact part number and ordering information.

Safety and Design Considerations

Working with 48 V is considered low voltage, but it is still a shock hazard in some environments. Follow standard safety practices:

• Always disconnect power before handling the board.
• Use proper insulation and creepage distances on the PCB for the 48 V rail.
• Ensure the module’s thermal protection is active; do not operate beyond the maximum junction temperature.
• Add input fusing and reverse-polarity protection if the supply is not already protected.
• Verify that your cooling solution can handle the module’s maximum power dissipation, especially at high ambient temperatures.

For a production design, consult the module’s reference design and application notes. The layout is critical – keep the high-current paths short and wide, and place the output capacitors close to the load.

Alternatives to the HP1800

If the HP1800 does not fit your application, consider these alternatives:

• Discrete buck converter – build your own with a controller IC and external MOSFETs; more flexible but more design effort.
• Other 48 V POL modules – many vendors offer similar modules with different current ratings, control interfaces, or package sizes.
• Intermediate bus converter (IBC) plus POL – use a fixed-ratio IBC to step 48 V to 12 V, then a standard POL; simpler but less efficient.
• Hybrid switched-capacitor converters – an emerging technology that uses capacitors instead of inductors for part of the conversion, potentially higher efficiency but less mature.

Each alternative has trade-offs in efficiency, cost, size, and design complexity. The HP1800 is a solid choice for direct 48 V to core power, but the best solution depends on your specific requirements.

Frequently Asked Questions

What is the typical input voltage range for the HP1800?

The HP1800 is designed for a nominal 48 V input, but it typically accepts 36–60 V to handle transients and tolerances. Always check the datasheet for the absolute maximum rating, which is often 60 V or 80 V.

Can the HP1800 be used for other voltages besides core power?

Yes, the HP1800 can be configured to output a range of voltages, usually from 0.6 V up to 5 V, making it suitable for other low-voltage rails like memory (VDDQ) or I/O. However, its high current capability is specifically optimized for core power.

Does the HP1800 require an external inductor?

No, the HP1800 is a complete module that includes the inductor and power switches. You only need to add input and output capacitors and the control interface connections.

How do I calculate the efficiency of my HP1800 design?

Measure the input power (V_in × I_in) and output power (V_out × I_out) under the same load, then divide output power by input power. The datasheet provides typical efficiency curves for reference.

What happens if the output current exceeds the HP1800’s rating?

The module may enter current limit or thermal shutdown. It is designed to protect itself, but sustained overcurrent can reduce reliability. Always derate the module for high ambient temperatures.

Is the HP1800 isolated?

No, the HP1800 is a non-isolated buck converter. The input and output share a common ground. If you need isolation, you would need a separate isolated DC-DC converter.

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in vin | Input voltage | V | 48 | 0 | 60
in vout | Output voltage | V | 1.0 | 0.1 | 5
in iout | Output current | A | 100 | 0 | 200
out pout | Output power | W | vout * iout | 2
out pin | Input power (90% eff) | W | (vout * iout) / 0.9 | 2
out iin | Input current | A | (vout * iout) / (0.9 * vin) | 2